Filament recoating apparatus and method

ABSTRACT

A filament coating apparatus comprising a frame including a first filament holding fixture and a second holding fixture for releasably securing a filament adjacent to an extrusion coating fixture also attached to said frame. The extrusion coating fixture comprises a guide rod, a carriage movably mounted on the guide rod, and a coating head including an opening for directing a curable coating composition to a filament positioned in a channel formed in the coating head. A radiation source is aligned to the coating head for radiation curing of the curable coating composition applied to the filament. Both the coating head and the radiation source are joined to the carriage to move relative to a portion of a filament during application of curable coating composition to its surface.

FIELD OF THE INVENTION

[0001] The invention relates to a process and equipment for convenientlyhandling a filament in the form of an optical fiber during multipleprocessing operations that may be at least partially automated. Moreparticularly the invention relates to compact handling of optical fibersduring manufacturing operations to include Bragg gratings in at least aportion of their length via a series of manufacturing operationsincluding mechanical stripping, acid stripping, Bragg grating writing,and optical fiber recoating and testing.

BACKGROUND OF THE INVENTION

[0002] Glass has been used for centuries as a material of choice in avariety of scientific and domestic applications. From the early use ofprismatic glass for separating light into its component colors, glasshas been widely used in optical devices that control or adjust theproperties of light beams. A recent and rapidly expanding application ofthe light modifying properties of glass structures involves the drawingof fine filaments of highly purified glass, more commonly referred to asoptical fibers, that direct light signals between light transmitting andreceiving locations.

[0003] During the late 1970's utilities began using optical fiberinstallations for internal communication, and by the early 1980's, anumber of small optical fiber networks were installed. The use of suchnetworks has been growing ever since to replace existing coaxial cablesystems. Advantages provided by optical fiber communications networksinclude lower cost, the use of fewer signal repeaters to correct forsignal distortion, and a higher signal carrying capacity than coaxialcable networks.

[0004] The capacity of fiber optic systems continues to increase. In1980, the first systems could transmit 45 megabits per second. Currentsystems transmit up to 5 gigabits per second. So extensive is the modernuse of optical fiber networks that fiber optic networks have essentiallyreplaced all transcontinental copper cable networks and entirely newnetworks are being created continually. One prediction claims that everycontinent in the World will become part of a global fiber optic network.

[0005] A fiber optic system includes three main parts of transmitcircuitry and light source, light detector and receiver circuitry, andfiber. The transmit circuitry converts electronic signals to modulate alight source that generates light signals for transmission. Connectionfrom the light source to a length of optical fiber facilitatestransmission of light signals for distances covered by the opticalfiber. Attachment of light detector and receiver circuitry at theterminal end of a fiber produces a communication link The use ofmultiple communication links provides extended networks of transmittersand receivers.

[0006] Interconnection of fiber optic networks requires high precisiondevices in the form of optical connectors that join optical fibers toperipheral equipment and other optical fibers while maintaining adequatesignal strength. In operation an optical connector centers the smallfiber so that the light gathering core lies directly over and inalignment with a light transmitting source or another fiber. Sections ofoptical fiber may also be spliced together using mechanical splicing orfusion splicing techniques.

[0007] Special features may be built into selected, relatively shortlengths of optical fibers to be spliced into fiber optic networks. Afiber Bragg grating represents a light-modifying feature that may beintroduced or written into an optical fiber by simple exposure toultraviolet light. The ability to write such gratings leads to a varietyof devices. For example, Bragg gratings may be applied intelecommunications systems to control the wavelength of laser light, tointroduce dispersion compensation, and, in the form of long periodgratings, to modify the gain of optical fiber amplifiers. Fiber opticapplications of fiber Bragg gratings, outside of telecommunications,include spectroscopy and remote sensing.

[0008] The process of introducing special features such as Bragggratings into an optical fiber may include a number of steps requiringhandling of relatively short lengths of optical fiber during a series ofmanufacturing operations. An optical fiber typically requires removal ofprotective coatings before changing the physical characteristics of thefiber to include a Bragg grating. After writing a Bragg grating, thefiber may be annealed and recoated.

[0009] Little has been revealed about the automation of processes toalter the characteristics of a fiber to provide it with a refractiveindex grating. Some evidence exists of individual processing steps butnot of a type that may be readily incorporated in an automated sequence.Fiber loading for example is described in U.S. Pat. No. 5,988,556. Thispatent refers to automated winding of a continuous length of fiber froma fiber supply onto first and second sections of a shipping spool. Thewinder comprises a first device that collects a first portion of acontinuous length of fiber and winds it onto the first section of thespool, and a second device for winding a second portion of thecontinuous fiber onto the second section of the spool. There is noevidence to show that the spooled fiber has a use other than as ashipping package. U.S. Pat. No. 6,027,062 describes an automated winderincluding fiber supply and collecting devices that move a fiber to athreading device that automatically threads the fiber onto a spool. Thisis similar to the goal of U.S. Pat. No. 4,511,095 to form of a coil offiber wound onto a bobbin or similar structure.

[0010] The use of stackable cassettes for handling and organizingoptical fibers is well known, particularly for storage of lengths ofspliced fibers. Cassettes typically comprise shallow dish-shaped holdersand enclosures for containment of loosely coiled optical fibers. Looseoptical fiber coils do not have the same compact structure as spooledoptical fibers. An intermediate form of coiled fiber, described in U.S.Pat. No. 5,894,540, may be produced using an assembly for holding alength of filamentary material in a wrapped configuration with a minimumbend radius. The filament or fiber may be wrapped around spools attachedto a support plate. Adjustment of the spacing between spools removesslack from the fiber wrapped around them. Fiber cassettes and relatedfiber holding assemblies place loose fiber in a tidy condition forstorage, usually following interconnection of lengths of optical fiber.U.S. Pat. No. 6,088,503 confirms the use of optical fiber cassettes asholders of optical fibers before, during and after splicing. The patentdescribes a clamping tool designed to align and hold a pair of fiberends in preparation for optical fiber splicing.

[0011] Cassettes and related fiber organizing assemblies provide tidystorage for optical fibers around connected and spliced sections ofoptical fiber. There appears to be no evidence of such storagecontainers used for processing organized lengths of optical fiber duringthe manufacture of optical fiber devices. One manufacturing processrequires the removal of protective buffers and coatings to reveal thebare surface of an optical fiber. Several processes are known forremoving protective layers, such as buffers and coatings, from thesurface of optical fibers. They include mechanical stripping, chemicalstripping and thermal stripping.

[0012] Mechanical stripping of optical fibers and related coatedfilaments requires careful positioning of sharp tempered metal blades toexpose a bare surface portion of a fiber without cutting or scratchingor otherwise physically damaging the fiber surface. Known methods ofmechanical stripping relate to cutting blade design and how a coatingmay be removed from the surface of a fiber. The predominant use ofmechanical stripping involves the removal of protective layers from theends of optical fibers, insulated wires and related filaments, prior tojoining the filament ends together. U.S. Pat. No. 4,434,554 describes anoptical fiber stripping device including a flat base having a number offiber receiving channels of suitable depth to ensure only removal of abuffer coating from each fiber, when a blade penetrates the coating. Theblade moves parallel to the axis of a fiber or group of fibers using aparing action to remove protective material. Channel size, based uponfiber diameter determines the selection of a flat base to provide adevice that strips a fiber end without damaging the fiber itself.

[0013] One way to avoid damage to the bare surface of an optical fiberrequires the use of blades designed to penetrate the protective bufferor fiber coating without reaching the fiber surface. Suitable bladeshave a substantially semicircular sharpened edge of a radius slightlylarger than the radius of the bare optical fiber. Two opposing blades,penetrating the protective layer around the fiber, interfere with eachother before the cutting edges reach the fiber surface. Afterpenetrating a protective layer, close to the end of a fiber, movement ofthe blades parallel to the fiber axis displaces a section of the layerto provide a bare fiber end untouched by the blades. United StatesPatents, including U.S. Pat. Nos. 4,630,406, 5,269,206, 5,481,638, and5,684,910, describe the manufacture and design of blades for cuttinginsulation from e.g. insulated electrical wires and optical fibers.Successful mechanical stripping using such blades may require additionaltreatments, including softening the protective layer as in U.S. Pat. No.5,481,638 requiring a chemical filled chamber first to soften anencapsulating layer then to clean plastic material from the blades afterstripping. U.S. Pat. No. 5,684,910 teaches an optical fiber havingimproved mechanical strippability. The improvement includes the use of afrangible boundary layer between a fiber coating and a buffer tofacilitate separation from the bare fiber. Previous teachings includeinitial blade movement perpendicular to a filament axis, to penetrate acoating, followed by movement parallel to the filament axis to exposebare filament ends by displacement of protective layers.

[0014] Chemical stripping may be used as an alternative to mechanicalstripping for preparing bare fiber ends. U.S. Pat. Nos. 4,865,411 and4,976,596 deal with controlled removal of coating, by gradual withdrawalof a coated fiber from a chemical bath, to produce a contoured shallowtaper adjacent to the bare glass fiber surface. A fixture, according toU.S. Pat. No. 5,451,294 provides support while dipping the end of acoated optical fiber into a chemical bath to dissolve coating from theend. Chemical stripping methods include common problems related to thehandling of chemicals especially, as in this case, when the chemicalstrippers involve corrosive liquids.

[0015] Hot gas stripping may be used instead of mechanical or chemicalstripping. One example of this process, described in U.S. Pat. No.6,123,801, uses a hot inert gas to melt buffer coating and blow it fromthe surface of an optical fiber. The process requires high pressure gasstreams and temperatures in the region of 800° C. to strip coating fromthe fiber. U.S. Pat. No. 5,939,136 describes a process for preparingoptical fiber devices including thermal removal of a coating from anoptical fiber, preferably performed using a heated gaseous stream.

[0016] A reason for removing protective buffers and related coatingsfrom optical fibers is the need to change the characteristics of thefiber such as by writing of a refractive index grating, also known as aBragg grating, in the core of an optical fiber. Refractive index changesoccur during exposure of a bare fiber to radiation from an ultravioletlaser or similar exposure device. The majority of protective coatingsfor optical fibers absorb the fiber modifying radiation. This explainsthe need to remove the coatings before writing a refractive indexgrating.

[0017] Without further processing, an optical fiber including arefractive index grating also has a bare portion that requiresapplication of protective coatings before use in an optical fiberdevice. The widely accepted method for recoating bare sections ofoptical fibers involves special coating molds. Methods similar to thoseused to coat drawn fibers, during their manufacture have also beendescribed.

[0018] A recoating mold, described in U.S. Pat. No. 4,410,561, providesa coated optical fiber using a split mold die structure. The size anddesign of a cavity formed by the closed mold provides space that becomesfilled during injection of curable, protective, fluid recoatingcompositions. It is desirable to avoid entrapment of air inside the moldsince this could lead to a defective recoated fiber section. Completefilling of a mold cavity may involve intentional application ofpressure. U.S. Pat. No. 5,022,735 uses a screw type plunger topressurize recoating fluid injected into a conventional recoating mold.Some recoating molds include curing means to provide finished recoatedsections of optical fibers. U.S. Pat. No. 4,662,307, for example, uses asplit mold including an injection port and UV light port through whichlight passes to cure recoating compositions. The curing process requiresmultiple light sources.

[0019] Application of coatings to an optical fiber drawn from a pre-formtypically places the emerging fiber in a vertical orientation. As ittravels downward, the fiber may pass through a reservoir of coatingfluid before exiting through an orifice sized to the desired externaldiameter of the coated fiber. It is possible to apply such a process torecoating of bare sections of optical fiber including a Bragg grating,as taught in U.S. Pat. No. 6,069,988. Upon exit from the orifice, thefiber moves past a source of curing radiation. The curing radiationdiffers from the radiation used for writing the Bragg grating so as notto destroy or change the characteristics of the grating.

[0020] There is evidence in Japanese Patents JP 60-122754 and JP61-40846 for spraying protective plastic coatings on optical fibersexiting a draw tower. Coverage of the full circumference of the opticalfiber requires the uses of either multiple spray heads or special spraycontainment shrouds. The use of multiple spray heads deposits only afraction of the spray on the surface of the drawn fiber while the use ofspecial shrouds involves complicated threading of a fiber.

[0021] Each point in the processes, of fiber stripping, modifying andrecoating, requires care to prevent damaging the fragile optical fiber.Damage to optical fibers may occur by physical contact or exposure totensile, torsional, twisting, and bending stresses. Excessive bendingcan change the optical characteristics of a fiber. Failure to meetrequired optical characteristics leads to rejection of an optical deviceand increases the expense of device manufacture. A need exists forimproved means for handling optical fibers for post draw processing, toreduce incidence of damage thereby reducing the cost and increasing theyield of optical fiber devices.

SUMMARY OF THE INVENTION

[0022] The present invention satisfies the need for effective andcompact handling of filamentary materials during manufacturingoperations including process steps that produce structural and relatedchanges in the filament. When applied to optical fibers, an article,also described herein as a filament organizer, provides compactcontainment of an optical fiber. The filament organizer allowsrelatively precise positioning of at least a portion of an optical fiberto facilitate processing of optical fibers related to optical couplers,fused couplers and tapered fiber devices and the like. Optical fibermodification may also refer to actions taken to change the inherentcharacteristics of an optical fiber or to incorporate an optical fiberinto a functional assembly. The inherent characteristics of an opticalfiber change with adjustment of its refractive index properties, as inthe formation of a variety of fiber Bragg gratings. Incorporation of anoptical fiber into a functional assembly provides useful devices such astemperature compensated fiber Bragg gratings. Refractive index changesand functional assembly production, according to the present invention,use a filament organizer that distributes an optical fiber between alockable spool and a rotary spool to expose a central portion of a fiberto be modified.

[0023] A computer controlled, or otherwise programmed, fiber dispensermay be used to load a prescribed amount of a substantially twist-freeoptical fiber between a pair of spools mounted on a common axis. Afterfiber loading the spools are separated, with fiber extending betweenthem, and mounted to a filament organizer for fiber storage and furtherprocessing. Use of computer controlled dispensing, combined with afilament organizer, allows accurate consistent loading and organizationof a selected length of optical fiber within the boundaries of thefilament organizer. Control of the loading process allows the productionof numerous holders containing approximately equal lengths of fiber,organized in similar fashion. After successful loading of an opticalfiber, a filament organizer provides a convenient article for handlingthe fiber through process operations required for the manufacture ofoptical fiber devices. Preferably the filament organizer includes meansfor applying a tension force between about 50 g to about 100 g to thefilament held therein.

[0024] A variety of devices use optical fibers that have beenstructurally modified to include in-line optical waveguide refractiveindex gratings in at least a portion of their length. Physical propertyvariation of gratings allows them to be tailored for specificapplications. In one embodiment, the present invention provides a fiberBragg grating obtained via a series of manufacturing operationsincluding mechanical stripping of an optical fiber, acid stripping,pigtailing, optical fiber Bragg grating writing, annealing and opticalmeasurement followed by recoating and testing. The final step oftesting, including fiber proof testing, confirms attainment ofperformance requirements desired of an optical fiber Bragg grating.

[0025] Each operation or step of the manufacturing process requiresattachment of one or more filament organizers to one or more filamentprocessors or apparatus designed specifically to accomplish a designatedstep. This requires that the size and shape of a filament organizerinclude aspects of design allowing convenient connection with severalfilament processors. As well as making suitable connection with severaltypes of filament processors, an important requirement of a filamentorganizer is containment of a prescribed length of filament that may beup to several meters in length. Preferably, in the case of an opticalfiber, a filament organizer holds most of the length of a filament on apair of spools leaving a portion of filament available for processing. Aspool holds two sections of optical fiber wound in the same direction onseparate sides of a divided spool core. One section of fiber extendsbetween a pair of spools while the other section of optical fiberprovides a pigtail portion that may be readily unwound from each spool.There is a pigtail section at each end of a continuous length of opticalfiber.

[0026] After winding a continuous length of optical fiber between a pairof spools and positioning the spools on a support board, fiber handlingmay proceed with reduced expectation of damage to the fiber. Also theuse of a filament organizer allows ready access to a portion of fiber.Ready access to this portion of fiber allows it to be modified initiallyby removal of protective coatings from its surface and thereaftersubjecting it to operations that change its physical and opticalproperties, as in the writing of a fiber Bragg grating into a baredportion of optical fiber. A filament organizer allows reproduciblepositioning of that portion of an optical fiber that will be modified.Reproducible positioning leads to predictable results of filament oroptical fiber modification by operations that may be conducted using aprocess where at least several of the steps may be automated.

[0027] As indicated previously, a filament organizer provides a portionof filament or optical fiber suitably positioned for processing.Formation of an optical fiber Bragg grating according to the presentinvention requires that any polymeric protective coating, also referredto herein as a buffer coating, should be removed prior to the writing ofthe fiber Bragg grating. The coating may be removed using liquid ormechanical or thermal stripping.

[0028] An optical fiber covered with a single polymeric layer, referredto herein as a primary buffer, may require only liquid stripping usingconcentrated acid to remove the buffer. Removal of multiple protectivecoatings, including primary and secondary buffers according to thepresent invention, preferably uses a combination of mechanical strippingfollowed by acid stripping. Acid stripping herein refers to dissolvingresidual coating material in an acid medium with displacement of theacid using a water rinse and solvent wash applied to at least a portionof the fiber. Initial displacement of coating requires speciallydesigned mechanical stripping equipment that cooperates with a filamentorganizer for precise positioning of the portion of an optical fiberfrom which protective coating will be displaced. Mechanical strippingequipment may be designed for conveniently processing one filamentorganizer or several combined in a single stacked configuration. Thisresults in treatment of one or more fibers at a time depending on thenumber of filament organizers. Coating displacement, via mechanicalstripping, creates gaps to the bare fiber through which acid maysubsequently penetrate to more rapidly dissolve coating from the fiberportion.

[0029] Removal of coating by acid stripping preferably requires anapparatus that forms a loop of filament for each filament organizerincluded in a stacked configuration. The apparatus is constructed forformation of individual filament organizer loops having approximatelythe same size. The plane of each loop parallels that of its nearestneighbors. Acid stripping of one or more fiber loops occurs by immersingthe arcuate portion of a loop into an acid bath. The depth of immersionof each loop into the acid bath controls the length of protectivecoating removed from a fiber to provide an optical fiber having a bareportion stripped to the silica surface of the fiber. Acid strippingprovides a bare fiber surface that is substantially free fromcontaminants.

[0030] After all of the fibers in a stacked configuration have beenmechanically stripped and acid stripped, the pigtail ends of each fiberare manually unwound and organized into groups using pigtail connectors.Pigtail ends trail about one meter from each end of a filamentorganizer.

[0031] As a further refinement, a filament organizer according to thepresent invention may include a conventional optical fiber connector forterminating optical fiber ends on the surface, and within the boundariesof the filament organizer. Optical connector termination of fibersreduces the length of pigtail portions of an optical fiber while stillproviding convenient points of attachment to external optical fiberdevices. Compact fiber organization of this type distributes the lengthof an optical fiber on the surface of a filament organizer without anypart of the fiber hanging over the edges of the organizer. Any of avariety of optical fiber device interconnects may be used to reduce theoverall length of an optical fiber by shortening the pigtail ends.Reduction in the overall length of an optical fiber translates into costsavings associated with each filament organizer equipped with pigtail tooptical fiber connector termination.

[0032] Following organization by grouping of pigtail ends each filamentorganizer in a stacked configuration provides a clean, dry, bare fiberportion ready for positioning in a fiber Bragg grating writingapparatus. After release of tension from a filament held by a filamentorganizer, the Bragg grating writing apparatus applies a selectedtension to the portion of an optical fiber before it is modified toproduce a Bragg grating. Production of multiple optical fiber Bragggratings, having a substantially identical wavelength response, requiresprecise alignment and application of the same amount of tension to eachoptical fiber portion loaded into the fiber Bragg grating writingapparatus. Precise alignment of an optical fiber portion with the Bragggrating writing apparatus relies on features built into a filamentorganizer and the grating writing apparatus respectively for consistentrelative positioning of one to the other. Consistent loading and fiberportion tensioning relies upon the use of a voice coil drive mechanismand air suspended bearings that facilitate accurate fine adjustmentessentially free from drag.

[0033] After placing an appropriate portion of an optical fiber undertension in the fiber Bragg grating writing apparatus, the progress ofBragg grating writing may be monitored by observing a display of thewavelength envelope produced by the writing process. Signal informationproceeds from an optical fiber to suitable monitoring equipment throughconnections between the equipment and pigtail ends of a fiber. Thisprovides feedback of the quality of a grating at the time of writing andrepresents a convenient decision point for acceptance or rejection afiber Bragg grating as it is written.

[0034] Annealing of fibers takes place in a thermal annealing apparatusand fulfils several requirements upon completion of writing of fiberBragg gratings. This step of the process proceeds at a temperature ofapproximately 300° C. for a duration of more than about three minutes.The annealing process stabilizes the Bragg grating against wavelengthdrift for time periods exceeding about twenty to about twenty-fiveyears.

[0035] After annealing and optical confirmation that the grating centerwavelength meets requirements, the fibers and associated Bragg gratingsare ready for recoating before final testing. The recoating operationuses equipment designed for a filament organizer or preferably a stackedconfiguration of filament organizers according to the present invention.It is possible to use in-mold recoating, spray recoating or an extrusiondie coating process to recoat the previously stripped portion of eachoptical fiber. Injection die coating refers herein to conventionalin-mold die recoating. Spray recoating uses multiple passes of anoptical fiber between a spray head and a radiation curing source. Theextrusion recoating process uses a split die that may be positionedaround an optical fiber for application of a curable coating compositionaround the circumference of the fiber as the extrusion head traversesthe length of an uncoated fiber portion. Preferably the die headincludes a radiation source and the extruded coating cures by exposureto the radiation source. This allows application of recoating materialfollowed immediately by curing.

[0036] Application of recoating material to protect a Bragg gratingformed in an optical fiber, represents the final processing operationfor producing fiber Bragg gratings that may be used intelecommunications and related applications. A final check of theresulting product determines if it passes tensile strength and visualinspection requirements. After successfully meeting requirements, thespools holding a finished optical fiber Bragg grating may be detachedfrom the filament organizer and used for conveniently holding, packagingand transporting the final product. A convenient form of packaging fortransportation requires transfer of the full continuous length of afiber Bragg grating to one spool after removing it from the filamentorganizer. The design of a spool provides a protective cover for thefiber Bragg grating element following transfer of the full length ofoptical fiber to one spool.

[0037] More particularly, the present invention provides a method formanufacturing an optical fiber refractive index grating. A suitablemethod comprises the steps of providing a substantially twist-freelength of an optical fiber between a first spool and a second spool, forattachment of the first spool and the second spool to a support. Thesupport has a first surface opposite a second surface, to provide afilament organizer including the first spool as a lockable spool and thesecond spool as a rotary spool. The filament organizer further comprisesa tensioner coupled to the rotary spool to apply tension to at least acentral portion of the length of an optical fiber disposed between thelockable spool and the rotary spool. Further processing of a fiber undertension includes removing at least a buffer coating from the centralportion of an optical fiber before applying a controlled tension to thecentral portion of an optical fiber. A refractive index grating may thenbe written by changing the refractive index characteristics of thecentral portion during exposure of the central portion to aninterference pattern of high intensity actinic radiation, to produce therefractive index grating. After formation the grating may be annealedand the resulting fiber device proof tested to confirm desiredperformance properties.

[0038] The method described previously uses a filament organizer,comprising a support having a first surface opposite a second surfaceand further including organizing mounts joined to said first surface andspacer blocks attached to said second surface. The filament organizerhas a lockable spool adjacent to the first surface of the support, arotary spool adjacent the first surface of the support, and a tensionerattached to the second surface of the support. The tensioner includes atension wire for attachment to the rotary spool to apply tension theretoto transmit tension to a filament disposed between the lockable spooland the rotary spool. A tension relief assembly allows selectivereduction of tension applied to a filament. The tension relief assemblyincludes the tension wire, providing connection between the tensionerand the rotary spool, a tension wire access, and at least one pulley foraligning the tension wire with the tension wire access. Other parts ofthe filament organizer include at least one mounting plate integrallyformed with the support and extending outwardly therefrom, and at leastone guide defining a filament path between the lockable spool and therotary spool. Further the guide is rotationally mounted on the mountingplate, adjacent to the first surface of the support, to provide spacingof the filament path from the support.

[0039] During refractive index grating manufacture a mechanicalstripping apparatus displaces resin from a resin covered filament, inthe form of an optical fiber, by forming a removable sleeve portionbetween opposing filament ends. The mechanical stripping apparatuscomprises a base that has a first clamp attached to the base to hold afilament at a first location. A second clamp is attached to the base andhas a separation from the first clamp and is in axial alignmenttherewith for holding a filament at a second location. The apparatusincludes a first set of cutting blades mounted on the base adjacent tothe first clamp. The first set of cutting blades includes a first upperblade and a first lower blade. Each of the upper and lower bladesincludes an arcuate sharpened edge for cutting into resin around a resincovered filament proximate to the first location. A second set ofcutting blades is mounted on the base adjacent to the second clamp suchthat a distance separates the first set of cutting blades from thesecond set of cutting blades. The distance between cutting blades isless than the separation between the clamps. The second set of cuttingblades includes a second upper blade and a second lower blade with eachblade including an arcuate knife edge for cutting into resin around aresin covered filament proximate to the second location. A bladeactuator secured to the base, and coupled to the first set of cuttingblades and the second set of cutting blades, moves the first upper bladeand the first lower blade together. During this movement the sharpenededges penetrate resin around a resin covered filament proximate to thefirst location. The blade actuator also moves the second upper blade andthe second lower blade together for the knife edges to penetrate resinaround a resin covered filament proximate to the second location. Abiasing component also on the base moves the first set of cutting bladesand the second set of cutting blades towards each other duringdisplacement of resin from a resin covered filament to form theremovable sleeve portion.

[0040] The removable sleeve portion may be formed using a method fordisplacing resin from a resin covered optical fiber between opposingfiber ends. The method provides a mechanical stripping apparatuscomprising a first clamp for holding an optical fiber at a firstlocation, a second clamp having a separation from the first clamp and inaxial alignment therewith for holding an optical fiber at a secondlocation. A first set of cutting blades, of the mechanical strippingapparatus, is adjacent to the first clamp for cutting into resin arounda resin covered optical fiber proximate to the first location. A secondset of cutting blades is adjacent to the second clamp for cutting intoresin around a resin covered optical fiber proximate to the secondlocation. A distance separates the first set of cutting blades from thesecond set of cutting blades. The distance is less than the separationbetween the first and second clamps. The first set of cutting blades andthe second set of cutting blades are adapted for movement towards eachother during removal of resin from a resin covered optical fiber to formthe removable sleeve portion. Resin displacement further includesclamping an optical fiber in the first clamp and clamping the opticalfiber in the second clamp such that the optical fiber is under tension.Operating the first set of cutting blades and the second set of cuttingblades, for cutting into the resin, produces the removable sleeve thathas a gap at each end thereof. The gap at each end exposes a barefilament portion separating the removable sleeve portion from a taperedtransition formed in the resin during cutting of the resin as the firstand second set of cutting blades move towards each other.

[0041] In another aspect according to the present invention an apparatusmay be used to form a loop in a section of a filament prior to chemicalstripping of resin from e.g. an optical fiber. The apparatus comprises acontainer including a front wall having a front guide slot formedtherein and a rear wall having a rear guide slot formed therein coplanarand parallel to the front guide slot. The container further includes afloor containing at least one slit formed between and parallel to thefront wall and the rear wall. A first filament gripper includes astationary elastomer roller and a positionable cylinder holding afilament therebetween, at a first location thereof. The stationaryelastomer roller is rotatably mounted from the front wall to the rearwall, so that the positionable cylinder is mounted, adjacent to thestationary elastomer roller, between the front guide slot and the rearguide slot for repositioning therein. A second filament gripper includesa movable elastomer roller and a movable cylinder holding the filamenttherebetween, at a second location. The second filament gripper has aseparation from the first filament gripper and has substantially axialalignment therewith. The second filament gripper moves towards the firstfilament gripper to reduce the separation to bring the first locationcloser to the second location thereby producing a loop of filamentbetween the first filament gripper and the second filament gripper. Theloop of filament extends through a slit to below said floor of thecontainer where it may be introduced into a reservoir having a solventtherein to surround at least a portion of the loop of filament todissolve resin from the portion of the loop. A loop forming containeraccording to the present invention may be sized to accommodate one ormore filament organizers having a filament between a lockable spool anda rotary spool. Steps for forming one or more filament loops using aloop forming container may be included in a process for chemicallystripping resin from a resin coated filament, preferably as an opticalfiber.

[0042] Processing of a filament according to the present inventionrequires the use of a filament holding fixture comprising a gripperhaving an open position and a closed position. The gripper furthercomprises a lower jaw mount, and a lower jaw connected to the lower jawmount, the lower jaw having a planar surface and an open-ended, V-shapedchannel formed therein opening to the planar surface to receive at leasta portion of a filament. The filament holding fixture also has an upperjaw mount, and an upper jaw assembly. The upper jaw assembly comprises asupport flange attached to the upper jaw mount. The support flangeincludes a support surface, having a substantially conical recessedportion. A fiber clasp, included in the upper jaw assembly, has acontact face opposite a structured surface. The structured surfaceincludes an open-ended groove of substantially rectangular crosssection. There is a substantially conical depressed portion formed inthe contact face of the fiber clasp. The open ended groove and theV-shaped channel are in longitudinal alignment to contact at least aportion of a filament when the gripper is in the closed position. Aplurality of spring connectors hold the fiber clasp to the supportflange. Also, an angular compensator is confined between the recessedportion of the support surface and the depressed portion of the contactface by force produced by the plurality of spring connectors. Theangular compensator maintains separation of the support flange from thefiber clasp to allow them to move independently. This leads to fineadjustment of the fiber clasp for applying substantially equal force atpoints of contact of the open-ended groove and the V-shaped channel witha filament, preferably an optical fiber, held therebetween followingmovement of the gripper from the open to the closed position.

[0043] The present invention further provides a filament tensioningapparatus for releasably securing a filament under tension. Thetensioning apparatus comprises a tensioning holder and a pair ofgrippers. The tensioning holder includes at least one support bar, and afirst carriage movably mounted at a first location on a support bar. Thefirst carriage includes an upper surface having a first clamp and avoice coil mounted thereon for movement relative to a support bar. Asecond carriage is movably mounted at a second location on a support barsuch that a separation exists between the first location and the secondlocation. The second carriage includes an upper face having a secondclamp and a load cell mounted thereon for movement relative to a supportbar. The second clamp is in axial alignment with the first clamp tosecure a measured filament portion including a bare portion thereof,located inside a first boundary and a second boundary, between the firstclamp and the second clamp. A guide bar extends from the voice coil forcontact with the load cell to adjust the separation of the firstlocation from the second location, to change tension applied to themeasured filament portion, during activation of the voice coil. The pairof grippers of the tensioning apparatus is in axial alignment with thefist clamp and the second clamp, to substantially immobilize the bareportion of the measured filament portion. A filament tensioningapparatus according to the present invention may include a coupling forattaching a filament organizer to position a filament, preferably anoptical fiber, to be held between the first clamp and the second clamp.The filament organizer holds a filament between a lockable spool and arotary spool.

[0044] A resin covered filament having had resin removed therefrom mayrequire coating by a method that uses a filament recoating apparatusaccording to the present invention. Such a filament recoating apparatuscomprises a frame for releasably securing a filament and a carriagemounted on the frame to oscillate between a first position and a secondposition. The recoating apparatus has a first filament holding fixturemounted on the carriage. A second filament holding fixture is alsomounted on the carriage in axial alignment with the first filamentholding fixture. The fixtures secure a measured filament portionincluding a bare portion thereof, located inside a first boundary and asecond boundary, between the first filament holding fixture and thesecond filament holding fixture. At least one spray head is attached tothe frame at the first position. A radiation source is attached to theframe at the second position. The measured filament portion movesbetween the spray head and the radiation source, during oscillation ofthe carriage between the first position and the second position to placethe bare portion to receive a curable coating from the spray head. Thespray head applies curable coating from the first boundary to the secondboundary. Curing of the curable coating occurs by exposure to radiationfrom the radiation source. Droplets of curable coating composition maybe deflected using a deflector, such as an air-knife, to selectivelydirect coating composition towards a plurality of bare filament portionsof filaments, preferably optical fibers, grouped around a spray head.Different coating compositions may be applied to bare filament portionsto provide recoated filaments using a first composition and overcoatedfilaments by application of a second coating composition over the firstcoating composition. The resulting filaments include a multilayercoating.

[0045] An alternative filament recoating apparatus, according to thepresent invention, comprises a planar surface and an extrusion coatingassembly attached to the planar surface. The extrusion coating assemblycomprises a first filament holding clamp and a second filament holdingclamp opposite the first filament holding clamp. A measured filamentportion including a bare portion thereof, located inside a firstboundary and a second boundary, lies between the first filament holdingclamp and the second filament holding clamp. A coating head, includes adie plate having formed therein an open ended channel including a wallhaving a fluid entry and a gas port formed therein adjacent a radiationsource. The coating head further includes a cover die plate havingformed therein an open ended elongate slot. The cover die plate has ahinged connection to the die plate for rotation of the cover die platebetween an open position and a closed position. In the closed positionthe cover die plate lies adjacent to the die plate and the channelaligns with the elongate slot to form a tubular opening through thecoating head to encircle a section of the bare portion. A lineartransport mechanism adjacent to the coating head includes a guide rodand a carriage slidably mounted thereon for movement along the guiderod. A connecting rod from the carriage to the coating head provideslinear displacement of the coating head during movement of the carriageto move the coating head from the first boundary to the second boundary.Curable fluid may be extruded from the fluid entry while energy from theradiation source cures the curable fluid to recoat the bare portion of afilament.

[0046] A method for extrusion coating a filament comprises the steps ofproviding a filament organizer having an extended filament between afixed spool and a rotary spool to provide a measured filament portionand a bare filament portion of a filament, preferably an optical fiber.Recoating of the bare portion of a fiber follows attachment of thefilament organizer to an extrusion coating fixture comprising a guiderod, a carriage movably mounted on the guide rod. A coating die,including a coating head and a radiation source, is joined to thecarriage. The coating head has an opening for directing a curablecoating composition to the bare filament portion positioned in a channelformed in the coating die and extending therethrough. A curable coatingcomposition is applied to the bare filament portion to provide arecoated filament portion, followed by exposing the recoated filamentportion to the radiation source for radiation curing of the curablecoating composition applied to the bare portion.

[0047] Definitions

[0048] The terms “bare fiber,” or “bare fiber portion,” or “strippedfiber,” or phrases relating to such terms refer herein to the portion ofan optical fiber from which protective coating has been removed toexpose the silica surface of the fiber.

[0049] As used herein, the term “cladding” refers to the outer layer ofan optical fiber, as drawn.

[0050] The term “buffer” or “primary buffer” refers herein to a polymeror resin layer next to a bare fiber.

[0051] A “coating” or “secondary buffer” is used herein to describe apolymer or resin layer next to a buffer or primary buffer.

[0052] The term “resin” as used herein is a general term describingpolymer coverings for filaments particularly optical fibers. Materialsused for previously defined buffers and coatings fall within the generalterm of resin.

[0053] The term “filament” herein refers to a fiber structure,preferably a “silica filament.” An optical fiber is a preferred form offilament according to the present invention.

[0054] A “tapered transition” describes the preferably graduated conicalshape of the portion of buffer layers closest to a bare fiber portionafter subjecting a coated optical fiber to mechanical strippingaccording to the present invention.

[0055] The term “ribbonizing” refers to the formation of a single layerof optical fibers, side by side, as a flat ribbon-like structure thatfacilitates the joining of ends of multiple fibers for insertion in oneend of a fiber optic ribbon connector.

[0056] The term “angular compensator” or “ball joint leveler” as usedherein means a self adjusting coupling inserted between parts of atleast one jaw of a gripper to achieve optimum positional relationshipbetween the contacting surface of the jaw and an object to apply evenpressure over the surface of the object.

[0057] The use of a “non-contact” method for recoating bare portions ofoptical fibers means that no portion of the fiber touches any part ofthe recoating equipment. This is a benefit of suspending a fiber in afilament organizer that may be readily attached to the recoatingapparatus with precise fiber to spray head alignment.

[0058] A “split sizing die” is a multi-part fiber recoating head thatopens to receive an optical fiber, closes to extrude curable recoatingmaterial around the surface of a length of fiber and re-opens to releasethe coated fiber.

[0059] The term “shroud” refers to a shield over an ultrasonic sprayhead to direct a stream of inert gas to entrain and move a cloud ofdroplets of recoating composition towards a target surface, such as abare portion of an optical fiber.

[0060] The present invention has been developed to provide a process andequipment for conveniently handling a filament in the form of an opticalfiber during multiple processing operations that may be at leastpartially automated as a further benefit to the user. These enhancementsand benefits are described in greater detail hereinbelow with respect tothe several aspects and alternative embodiments of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0061]FIG. 1 shows a perspective view of a filament organizer accordingto the present invention.

[0062]FIG. 2 provides detail portions of a filament organizer accordingto the present invention in exploded perspective view

[0063]FIG. 3 shows a perspective view of an assembly for applyingtension to a filament contained in a filament organizer according to thepresent invention.

[0064]FIG. 4 is a side elevation of a filament organizer.

[0065]FIG. 5 provides an perspective view of a plurality of filamentorganizers in a stacked configuration.

[0066]FIG. 6 is a cross sectional view through a dual coated opticalfiber.

[0067]FIG. 7 shows a cross section of a dual coated optical fiber afteracid stripping.

[0068]FIG. 8 provides a cross section of a dual coated optical fiberafter mechanical stripping to provide a tapered transition.

[0069]FIG. 9 shows a side elevation of an optical fiber after mechanicalstripping to produce a separated central buffer sleeve.

[0070]FIG. 10 is a cross sectional view showing a coated optical fiberpositioned for mechanical stripping.

[0071]FIG. 11 is a cross sectional view showing a coated optical fiberafter formation of a tapered transition

[0072]FIG. 12 shows a perspective view of a cutting blade according tothe present invention for use during mechanical stripping of coatingfrom an optical fiber.

[0073]FIG. 13 provides a detail view of a cutting edge of a cuttingblade according to the present invention.

[0074]FIG. 14 is a detail view showing the relative positioning of acoated optical fiber and the cutting edge of a cutting blade accordingto the present invention.

[0075]FIG. 15 is a cross sectional view indicating the depth of cut of acutting edge, and positioning of an upper blade that has penetrated asecondary buffer around an optical fiber.

[0076]FIG. 16 provides a cross sectional view showing depth of cut of anupper blade and a lower blade during mechanical stripping of secondarybuffer from an optical fiber.

[0077]FIG. 17 is a diagrammatic representation of a side elevationshowing the relative positioning of a filament organizer and mechanicalstripping apparatus according to the present invention.

[0078]FIG. 18 provides a cross sectional view of a coated optical fiberloop during removal of coating by immersion in acid contained in an acidbath.

[0079]FIG. 19 is partial cross section showing the relative positioningof a filament organizer according to the present invention and anacid-containing bath before formation of an optical fiber loop.

[0080]FIG. 20 is partial cross section showing the relative positioningof a filament organizer according to the present invention including aloop suspended from the filament organizer for immersion of an arcuateportion of the loop below the acid surface.

[0081]FIG. 21 is a perspective view including a cutaway section to showinternal detail of a loop former for a stacked configuration of filamentorganizers.

[0082]FIG. 22 provides a side elevational view of a filament organizeraccording to the present invention wherein pigtail portions of filamenthave been unwound from storage reels.

[0083]FIG. 23 is a diagrammatic side elevation of a filament organizeraccording to the present invention positioned in a voice-coil tensioningdevice during modification of a stripped portion of an optical fiber.

[0084]FIG. 24 provides a cross sectional view of a jaw used to preventmovement of a portion of an optical fiber while it undergoesmodification.

[0085]FIG. 25 is a detailed cross sectional view of the structure of thejaw shown in FIG. 24.

[0086]FIG. 26 is a perspective view of a jaw assembly according to thepresent invention.

[0087]FIG. 27 is a cross section taken through line 27-27 of FIG. 26.

[0088]FIG. 28 is a diagrammatic side elevation of a filament organizeraccording to the present invention positioned in a spray recoatingapparatus.

[0089]FIG. 29 is a diagrammatic side elevation of a filament organizeraccording to the present invention positioned in a split die recoatingapparatus.

DETAILED DESCRIPTION OF THE INVENTION

[0090] Referring to the Figures wherein like numbers refer to like partsthroughout the several views. FIG. 1 is a perspective view of a filamentorganizer 10 including a substantially planar support board 12 that hasopposing sides. The support board 12 has, on its first side 14, alsoreferred to herein as its upper side, points of attachment for alockable spool 16 and a rotary spool 18. Theses spools 16, 18 have alength of filament 20 wound around them and between them. The supportboard 12 may include mounting plates 22 for one or more guides 24 usedto establish a preferred position for a portion of a filament 20extending from the lockable spool 16 to the rotary spool 18. Attachmentof a guide 24 to a mounting plate 22 allows suitable movement of theguide in response to movement of the filament 20.

[0091] A support board 12, according to the present invention,optionally includes at least one opening 26 formed therein as aconvenient location for holding a support board 12 by hand. An opening26 preferably occupies a location sufficiently separated from an opticalfiber 20 to prevent inadvertent touching of the surface, especially abare surface of a filament 20, such as an optical fiber. The positioningof an opening 26 has suitable separation from a filament when locatedbetween the lockable spool 16 and rotary spool 18 opposite an exposedportion of a filament 20, as illustrated in FIG. 1.

[0092]FIG. 2 uses a partially exploded view to show detail of anembodiment of a filament organizer 10 according to the presentinvention. This view depicts how a lockable spool 16, a rotary spool 18,and a pair of guides 24 may be attached to a substantially planarsupport board 12. As suggested earlier, a computer controlled fiberdispenser may be used to load a prescribed amount of optical fiberbetween a pair of spools 16, 18. A continuous length of fiber preferablyhas three sections including a main or central section, several metersin length separating opposite end or pigtail sections, eachapproximately one meter long. Typically the central section has a lengthof from about one meter to about six meters. Each of the spools 16, 18provides storage for an optical fiber pigtail. Filament turnsrepresenting pigtail sections are wound on a spool 16, 18 in the samedirection as the filament turns of the central section. The spools 16,18 each include a core divider (see FIG. 4) to separate pigtails fromthe central section of an optical fiber 20 to facilitate unwinding ofpigtails during filament processing.

[0093] After fiber loading, the spools 16, 18 are separated and mountedto a support board 12 for fiber storage with a length of filamentstretching between the spools 16, 18. One of the spools 16, 18 becomes alockable spool 16 during sliding engagement between an axial channel 28in the spool 16 and a post 30 secured to the support board 12. Eachfaceplate 32 of the lockable spool 16 includes at least a pair ofopenings 34 positioned on either side of the axial channel 28. Theopenings 34 align for engagement with a pair of pegs 36 rigidlyconnected to the support board 12. When the axial channel 28 andopenings 34 seat over the post 30 and pegs 36 the lockable spool 16cannot rotate since the pegs 36 restrict its movement. After mountingthe lockable spool 16, as described previously, a change in length offilament 20 between the lockable spool 16 and the rotary spool 18requires adjustment of a filament 20 by rotating the rotary spool 18.Rotation of the rotary spool 18 relies upon a bearing 38 held byfriction in a hub orifice 40 formed in the support board 12 of thefilament organizer 10. The bearing 38 facilitates rotation of a spoolhub 42 on one side and tensioning hub 44 on the other. The spool hub 42has a spindle 46 and a pair of pins 48 for alignment with an axialopening 50 and receiving holes 52 in both faceplates 54 of the rotaryspool 18. Although held in fixed relationship, the rotary spool 18 andspool hub 42 have rotational freedom provided by the bearing 38.

[0094] The embodiment of the present invention illustrated in FIG. 2includes a pair of guides 24 each taking the form of an idler wheel thatrotates around an axle 56. One end of the axle 56 connects to the firstside 14 of the support board 12. Filament 20 un-spooling, by rotation ofthe rotary spool 18, produces sufficient filament 20 between thelockable spool 16 and the rotary spool 18 so that the path of thefilament 20 from the lockable spool 16 to the rotary spool 18 passesaround each of the idler wheels 24. The resulting assembly, includingthe filament 20, provides a preferred orientation, as shown in FIG. 1,when the filament 20 is an optical fiber. This orientation places anoptical fiber 20 in a readily accessible, spatially precise position forprocessing, during the manufacture of a fiber Bragg grating, forexample.

[0095] Formation of an optical fiber Bragg grating frequently requiresapplication of tension to a filament 20, i.e. optical fiber, held by afilament organizer 10 according to the present invention. Reference toFIG. 3 indicates one means to apply tension to a filament 20 using atensioner 58 attached to the lower side 60 of a support board 12. Thetensioner 58 applies a constant force to a filament 20 or optical fiberto maintain the filament 20 under slight tension. Force from thetensioner 58 may be transmitted to the filament 20 through a series ofcomponents including a tension wire 62 connected between the tensioner58 and the tensioning hub 44. The tensioning hub 44 acts upon the spoolhub 42 because of a direct connection between the two. Movement of thespool hub 42 causes corresponding turning movement of the rotary spool18 which is centered on the spindle 46 and driven by the movement ofpins 48 attached to the spool hub 42. Movement of the rotary spool 18produces tension in the filament 20 or optical fiber proportional to theconstant force produced by the tensioner 58.

[0096] The tension in the tensioner 58 acts on the tension wire 62 witha force of from 20 g to 200 g. Force distribution through the tensionwire 62 and the tensioning hub 44 leads to a resultant force of about 50g to about 100 g of tension in a filament 20 held in a filamentorganizer 10. A tensioning hub 44 is typically about half the diameterof a rotary spool 18.

[0097] The tension wire 62 may pass unimpeded between the tensioner 58and the tensioning hub 44. Preferably, however, the location of asection of the tension wire 62 allows access and positional adjustmentof the tension wire 62 to remove tension from the filament 20. Removalof the effect of the constant force tensioner 58 uses a force reductionassembly comprising a pair of pulleys 64 on either side of a notch 66formed in an edge of the support board 12. The tension wire 62, passingaround the pulleys 64 and across the notch 66, may be grasped in thevicinity of the notch 66 and extended slightly parallel to the edge ofthe board 12, and toward the nearest guide 24. Movement of about 1.0 mmto about 2.0 mm releases the tension force applied to a filament 20 bythe tensioner 58. Release of tension relaxes a filament 20, in the formof an optical fiber, in preparation to re-tension the fiber. Relaxationand re-tensioning of an optical fiber 20 applies a known and repeatableamount of tension required by the Bragg grating writing process so thatresulting optical fiber Bragg gratings will exhibit substantially thesame wavelength response.

[0098] After installing a filament 20 under tension on a filamentorganizer 10, a length of filament several meters long may beconveniently carried, in a protected spooled condition, between handlingstations, during filament processing. Earlier methods for filamentprocessing required an operator to hand-carry lengths of filamentmeasuring in excess of six meters. Care was required to avoid contact oftrailing filament with the floor and other surfaces that could causecontamination and reduction in the yield of manufactured filamentdevices. A filament organizer 10 according to the present invention maybe handheld using the opening 26 in the support board 12. The positionof the opening 26 minimizes undesirable inadvertent hand contact withany exposed portion of the filament 20.

[0099]FIG. 4 shows a side elevation of a filament organizer 10 accordingto the present invention to indicate the relative positioning ofcomponents on the upper 14 and lower 60 side of a support board 12. Thefigure shows the tensioner 58, spacer blocks 70, tensioning hub 44,tension wire 62 and a pulley 64 on the lower side of the support board12. The upper side of the support board 12 provides a surface forattachment of organizing mounts 72, a lockable spool 16 and a rotaryspool 18 having a filament or optical fiber 20 held under slight tensionbetween them, as described previously. The spools 16, 18 used to store alength of optical fiber 20 are divided spools since the lockable spool16 includes a dividing wall 15 and the rotary spool has a partitioningwall 17. Part of an optical fiber 20 lying between the spools 16, 18occupies a lower core 19 of a spool 16, 18 below each of the dividingwall 15 and the partitioning wall 17. This leaves the upper core portion21 of each spool, i.e. above the dividing wall 15 and the partitioningwall 17 to receive a pigtail end of a continuous length of optical fiber20. Separation of a central length from the ends or pigtails of anoptical fiber 20 using lower 19 and upper 21 core portions of storagespools 16, 18 facilitates unwinding of only the pigtail portions of afiber 20 during fiber processing.

[0100] With suitable design, two or more filament organizers 10 may becombined to form an assemblage of filament organizers 10. The term,stacked configuration 68, describes herein an assemblage of filamentorganizers 10, as illustrated in FIG. 5. Design considerations includethe placing of stabilizing spacer blocks 70 on the lower surface 60 of asupport board 12 for mating registration with organizer mounts 72positioned on the upper surface 14 of a support board 12. The spacerblocks 70 may have insufficient height to hold a tensioner 58 clear of aplanar surface upon which a filament organizer 10 may be placed prior toforming a stacked configuration 68 thereon. This problem may be overcomeusing a suitably contoured spacer between the lower surface 60 of asupport board 12 and the planar surface. Correct stack 68 formationrequires addition of filament organizers 10 one on top of another withsuitable alignment of downward facing spacer blocks 70 and upward facingorganizer mounts 72 to produce a stable stacked configuration 68 bymating registration between filament organizers 10. The combined heightof spacer blocks 70 and organizer mounts 72 provides sufficient spacingbetween filament organizers 10. Alignment of spacer blocks 70 andorganizer mounts 72 produces a stacked configuration 68 neatlyorganizing a number of filaments 20, corresponding to the number offilament organizers 10, in a pre-determined relationship. Thisrelationship facilitates optimum orientation of a stacked configuration68 with filament processing equipment. The spacing between filaments 20in a stacked configuration 68 is from about 12.5 mm (0.5 inch) to about27.5 mm (1.2 inches) preferably about 18 mm (0.7 inch) to about 23 mm(0.9 inch).

[0101] Spacer blocks 70 and organizer mounts 72 may be viewed as primarycomponents for aligning one filament organizer 10 relative to itsnearest neighbor. Actual fiber 20 positioning and spacing depends uponthe location and relative height of the spacer blocks 70 and theorganizer mounts 72 on a support board 12. This means that the design ofa support board 12 determines the position of a length of filament 20 sothat it may be readily located in a stacked configuration 68. Inaddition to this, a filament 20 occupies a known position on a supportboard 12 with consistent spacing of the fiber 20 from other portions ofthe support board 12, such as the mounting plates 22 and upper side 14and lower side 60 of a support board 12. These features providereference points for uniting filament organizers 10, 68 to variouspieces of apparatus with one or more filaments 10 positioned ready forprocessing. This provides a filament organizer 10 and a stackedconfiguration 68 suitable for use in automated filament processingwithout operator intervention.

[0102] Means for handling multiple filament organizers 10 in a stackedconfiguration 68 includes installation of a stacking connector 74 thatoptionally includes a carrying handle 76. Preferably a stackingconnector 74 comprises one or more rods 78 inserted into through-holes80 (see FIG. 1) included in a support board 12. A rod 78 may include aflange as a support for the lowest filament organizer 10 in a stackedconfiguration 68. Alternatively, when a stacking connector 74 comprisesadditional rods 78 threaded through multiple filament organizers 10, abracket may be used to connect the ends of rods 78 adjacent to the lowerface 60 of the lowest filament organizer 10 in the stack 68. A carryinghandle 76 may be attached to a stacking connector 74 by any of a varietyof joining methods. Rods 78 protruding from a stacked configuration 68may include threaded end portions that may be received in tubularopenings (not visible) in a carrying handle 76. A threaded nut orsimilar retainer 82 may secure the handle 76 to the stacking connector74. This provides a stable stacked configuration 68 of multiple filamentorganizers 10 that may be carried with ease between stations forprocessing a plurality of filaments 20 in a single batch or automatedoperation.

[0103] A filament 20 in the form of an optical fiber, once installed ina filament organizer 10, requires a series of steps to producerefractive index modifying features in a selected portion of the opticalfiber 20. Optical fiber manufacture, using a draw tower, typicallyincludes the application of protective coatings over the length of thefiber. Identification of protective coatings for optical fibers uses avariety of terms including buffer and coating. The term buffer usuallyidentifies a material coated directly on a bare optical fiber. A coatingusually designates a protective material coated over a buffer layer.

[0104]FIG. 6 is a cross sectional view of an optical fiber showinglayers of protective coatings. As used herein the term primary bufferrefers to a buffer coating 100 and the term secondary buffer 102 refersto a coating applied to the primary buffer 100. Optical fiber Bragggrating manufacture requires the removal of both the primary buffer 100and the secondary buffer 102 from a central portion of an optical fiber20 stored on a filament organizer 10. One method for stripping a buffercoat requires dipping the fiber in a hot concentrated sulfuric acidbath. Preferably the sulfuric acid concentration is at least 95% beforeheating and stripping the buffer coating 100, 102 from an optical fiber.Acid stripping occurs at a temperature above about 150° C. Damage to aglass core 106 is less likely to occur with acid stripping than withother methods used to remove buffer coats 100, 102, since glass isresistant to acid.

[0105] A hot acid bath provides an effective medium for removing asingle buffer coat, but some types of optical fiber have multiplecoatings that may dissolve at different rates. These types of opticalfiber may include a relatively insoluble, hard secondary buffer coating102 over a softer protective primary buffer coating 100, as illustratedin FIG. 6 and FIG. 7. During hot acid stripping, the softer primarybuffer coating 100 may dissolve faster than the secondary buffer coating102. The process of dissolving polymer layers from an bare optical fiber106 may be accompanied by decomposition due to depolymerizingsulfonation caused by the attack of the concentrated sulfuric acid.Polymer decomposition products may impair the appearance and performanceof a modified optical fiber according to the present invention.

[0106] Placement of the primary buffer coating 100 under the secondarybuffer coating 102 can result in preferential removal by acid of theprimary buffer coating 100. Preferential removal of the primary buffercoating 100 produces an undercut 104 below the secondary buffer coating102 that can collapse inward towards the bare optical fiber 106. As thesecondary buffer coating 102 collapses it can trap acid or air bubblesnext to the bare optical fiber 106. Entrapment of material includingacid, and other liquids or gases, can produce conditions leading topremature failure of an optical fiber 20 for intended applications.

[0107] A contributor to premature failure, as indicated previously, maybe the existence of decomposed polymer species after strong acidtreatment. This situation may be avoided using, an intermediate,mechanical stripping method to provide a cut tapered transition section108, as shown in FIG. 8, between the primary coating 100 and an bareoptical fiber 106. The tapered transition 108 prevents the collapse of asecondary buffer coating 102, as previously described. When mechanicalstripping, according to the present invention, precedes acid strippingthe formation of a tapered end 108 can present a preferred geometry atthe junction between the primary buffer coating 100 and the bare surfaceof an optical fiber 106.

[0108]FIG. 9 shows how an intermediate, central portion of an opticalfiber 20 may be stripped from two points along its length using a pairof cutting blades. The intermediate portion to be stripped preferablyresides in a filament organizer 10 according to the present invention. Amechanical optical fiber stripping apparatus accommodates a filamentorganizer 10, providing correct orientation for stripping buffercoatings 100,102 from an optical fiber 20. The apparatus controls blades(not shown) that cut into the secondary buffer coating 102 to produce aseparated buffer sleeve 110 between a pair of tapered transitions 108that define the length of bare optical fiber when the primary 100 andsecondary 102 buffer coatings have been removed. Each end of the buffersleeve 110 includes a peeled-back collar 112 that provides a gap foraccess to the bare surface of the optical fiber 106.

[0109]FIG. 10 and FIG. 11 provide clarification of the basic componentsand steps required for stripping a central portion of a coated opticalfiber 20. A first clamp 120 holds one end of a portion of a coatedoptical fiber 20. The coated optical fiber 20 comprises a fiber core 106overcoated with one or more protective resin layers 100, 102. A secondclamp 122 holds the other end of the portion of the coated optical fiber20. Both clamps 120, 122 grip the outer surface of a relatively hardsecondary buffer coating 102. This prevents damage to the underlyingoptical fiber core 106. Preferably the clamps 120, 122 includefrictional gripping surfaces such as rubber or elastomer grippingsurfaces that resist fiber movement during mechanical stripping.

[0110] The immobilized coated optical fiber exists under slight tension,preferably of about 50 g. Typical separation between the first clamp 120and the second clamp 122 is from about 50.0 mm (2.0 inches) to about 100mm (4.0 inches) preferably 75.0 mm (3.0 inches) to about 90 mm (3.5inches). After limiting optical fiber 20 movement between a pair ofclamps 120, 122 at least one set of cutting blades 124 may be placedabutting the coated optical fiber 20 with the sharp edge of an uppercutting blade 126 resting against the surface of the coating 102surrounding the optical fiber 20. The desired position is shown by thelocation of a first set of cutting blades 124 relative to the clamped,coated optical fiber 20. A second set of cutting blades 130 is shown inFIG. 10 in a position, adopted by the cutting blades 130, afterpenetration of the secondary buffer 102 of an optical fiber 20. Each setof cutting blades includes an upper blade 126 and a lower blade 128. Thesharp edge of each cutting blade 126, 128 includes at least one notchhaving a radius in common with any primary buffer coat 100 applied to anbare optical fiber 106. During fiber stripping, the upper 126 and lower128 blades move inwards, as shown for the second set of cutting blades130, cutting through the secondary buffer 102 until they touch oneanother, before penetrating the primary buffer 100. The distance betweenthe first set of cutting blades 124 and the second set of cutting blades130, at this point, is typically between about 30 mm (1.2 inches) andabout 40 mm (1.5 inches). After cutting through an outer or secondarybuffer coating 102, application of suitable force moves the second setof cutting blades 130 closer to the first set of cutting blades 124 andparallel to the axis of the optical fiber 20. This transverse movementof the second set of blades 130 generates a stripping action thatresults in a gap 132 in the coating around the optical fiber core 106.The stripping action exposes a bare fiber portion 106 in the gap 132.One side of this gap 132 has a contoured, tapered transition 108, in theprimary buffer 100, from the bare optical fiber 106 to the secondarybuffer coating 102. The other side of the gap includes a compressed,peeled-back collar 112 of stripped coating 100, 102. When the cuttingand stripping operations have been completed at one end of the coatedoptical fiber portion, the opposite end of the fiber 20 may be strippedby initiation of cutting action of the first set of cutting blades 124.This produces a second gap 134 in the coating around the bare fiber 106,as illustrated in FIG. 9. The second gap 134 includes a similar taperedtransition 108 to that produced by the cutting action of the second setof cutting blades 130.

[0111] Application of acid stripping to a mechanically stripped fiber,as in FIG. 9, preferably exposes only the buffer sleeve 110 to acidattack. As long as the tapered ends 108, also referred to herein astapered transitions, remain out of the strong aqueous acid they remainfree from attack and chemically unchanged. In this condition the taperedtransitions 108 have a surface energy more compatible with recoatingcompositions. This allows the recoating compositions to readily wet thesurface of the tapered transitions 108 following modification of thecentral portion of a filament. Surface compatibility and ready wettingby recoating compositions produces defect free junctions betweenpreviously coated and recoated sections of optical fibers. The existenceof defects, e.g. air bubbles, in transition areas of an optical fibermay adversely affect the mechanical strength and light transmissioncharacteristics of an optical fiber device, rendering it unsuitable forits intended use.

[0112]FIG. 12 shows the design of blade components used for cutting thesecondary buffer coating 102 and displacing the primary buffer coating100 to cause separation of the primary buffer coating 100 from a bareoptical fiber 106. As illustrated in FIG. 12, a blade 140 providesdetail of features that may be included in both sets of cutting blades124, 130. The blade includes provision for stripping several opticalfibers 20 simultaneously. It will be appreciated that the same blade 140may be used to strip single or multiple fibers 20 depending on thenumber of filament organizers 10 inserted into the stripping apparatus(see FIG. 17).

[0113] A stripping blade 140 according to the present invention includesat least one bevel 142 as a portion of the blade 140 that includesseveral channels 144 machined into its surface. The channels 144 open toan edge 146 of a bevel 142 as sharpened notches 148 having approximatelycircular cross-section when viewed from the side opposite the bevel 142,as in FIG. 14. A detail view, shown in FIG. 13 provides clarification ofthe structure of the bevel 142 including the channels 144 machinedtherein. A coated optical fiber 20 is included in FIG. 14 to indicateits preferred position before penetration of the secondary buffer 102 bya sharpened notch 148 of a cutting blade 140. The knife-edge of asharpened notch 148 preferably reaches only towards the outer surface ofthe primary coating 100 of an optical fiber 20 without cutting into it.When used for stripping coating from an optical fiber 20, the notches148 cut a circular path around an optical fiber core 106 as shown inFIG. 15 and FIG. 16. In the illustration of FIG. 15 a sharpened notch148 appears as it would after an upper cutting blade 126 penetrates thesecondary buffer 102 of an optical fiber 20. This relates to theposition of the second set of cutting blades 130 as shown in FIG. 10.The relationship between the upper blade 126 and lower blade 128 of thesecond set of cutting blades 130 appears in the diagrammaticrepresentation shown in FIG. 16. The sharpened notches 148 of the upperblade 126 and lower blade 128 have penetrated the secondary buffercoating 102 without reaching the surface of the primary buffer coating100. Since the advancing edges 146 of the blades 126, 128 have madecontact there can be no further advancement of either blade 126, 128.

[0114] Stripping blades 126, 128 according to the present inventionperform biaxial movement. Initial movement of a blade 126, towards afiber core, produces a cut as a blade penetrates the secondary buffercoating 102 of the fiber 20. After traveling the thickness of thesecondary buffer coating 102, the blade begins to move toward the centerof the optical fiber 20, parallel to its axis. This movement disruptsthe coating 100, 102, producing a taper 108 clearly visible in thesofter primary buffer coating 100. In certain cases, the taper may alsoextend into the secondary buffer layer as shown in FIG. 11. The taper108 provides a conical boundary separating the bare optical fiber 106from the overlying buffer structure 100,102.

[0115] As described, the mechanical stripping apparatus includes twosets of vertically opening and closing cutting blades 124, 130 adaptedfor vertical, then horizontal movement either independently orsimultaneously. A pair of clamps 120, 122, on either side of the cuttingblades 124, 130, holds a strippable filament in a taught conditionduring the stripping process. Another embodiment of a mechanicalstripping apparatus alters the angle of the incision during the cuttingprocess to modify the shape of a tapered transition 108. As the blades124, 130 close towards the coating 100, 102 around a clamped fiber 20 anangled surface or biasing cam surface deflects the path of the blades toa prescribed entry angle into the coating 102 so as to provide acontrolled tapered transition. This produces an intentionally angled cutby moving the blades 124, 130 diagonally into the coating. Any change inthe angle of the cam surface produces a corresponding change in theangle of a tapered transition 108 to allow consistently reproduciblecontours of a coating 100, 102 abutting either side of a bare portion ofan optical fiber. Suitable selection of the cam angle produces taperedtransitions 108 having contours and dimensions facilitating essentiallydefect-free recoating of bare optical fiber portions. Successfulmechanical stripping to provide a tapered transition may proceed underambient conditions, as indicated previously. With some buffers, however,the modulus of the buffer resin is in a range that complicates theformation of a tapered transition. In such cases, it may be necessary tosoften the resin by heat or chemical action before attempting themechanical stripping process to produce the desired tapered transition.

[0116] Completion of the mechanical process of fiber stripping leaves acentral portion of an optical fiber 20 having a central sleeve 110 ofprotective buffer coating 100, 102 that has been separated from theremainder of the buffer coating 100, 102 over the optical fiber core106. Opposing gaps 132, 134 between the sleeve 110 and remainder of thebuffer coating 100, 102 provide points for hot acid to penetrate underthe central sleeve 110 to facilitate removal of the sleeve 110, whichdissolves in hot acid. Preferably the acid does not reach the taperedtransitions 108 of a previously mechanically stripped coated opticalfiber 20. Removal of the central sleeve 110, as a solution in hot acid,followed by rinsing in water and alcohol, leaves a clean, bare portionof an optical fiber 106 in suitable condition for further processing.Depending on the effectiveness of mechanical stripping, much of thedisrupted buffer sleeve 110 may be lifted from the bare fiber portion.This reduces the amount of buffer coating to be dissolved from a fiber20 during acid stripping.

[0117] Stripping of protective buffer coating from an optical fiber 20may be conducted as an automated or semi-automated process usingequipment suitably designed for the task. Preferably design of theequipment allows processing of multiple fibers 20 in a single operation.FIG. 17 shows the positioning of a filament organizer 10 relative to amechanical stripping apparatus 150. A mechanical stripping apparatus 150according to the present invention includes a base 152 as a mountingplatform for optical fiber clamps 120, 122 and sets of cutting blades124, 130. Optical fiber clamps 120, 122 may either move relative to thebase 152 or be secured thereto. Optional securing of the clamps 120, 122facilitates mechanical stripping with a fiber 20 either at its originaltension, set by the filament organizer 10, or under tension produced bymoveable clamps 120, 122.

[0118] The sets of cutting blades 124, 130 slidably engage the surfaceof the base 152. Slidable engagement of the sets of cutting blades 124,130 facilitates the axial movement of the blades 124, 130 to form atapered transition 108, as previously described. A filament organizer 10may be suspended by any suitable method relative to the mechanicalstripping apparatus 150 provided that the filament 20, clamps 120, 122and cutting blades 124,130 have alignment on a common axis.

[0119] After the preliminary step of mechanically stripping the centralportion of a fiber 20, acid stripping may require formation of a loop,suspended from a filament organizer 10. The suspended loop may besubmerged in hot concentrated sulfuric acid. An acid bath is aconvenient and clean method to remove the buffer coat fromacid-resistant glass.

[0120] A known method uses acid to remove protective coatings fromoptical fibers. The method requires handling of fibers each individuallyas much as six to eight meters long. Handling of such optical fibersrequires caution because of the small diameter and transparency of thefilamentary structure. If the optical fiber snags an object duringhandling, the glass fiber core could fracture without showing immediateevidence of damage.

[0121] A manual method for loop formation includes extending an opticalfiber over two blocks having a distance of separation of about sixinches. The folding of a first block 160 over a second block 162produces a loop 164, shown by the diagram of FIG. 18. This figure alsoincludes an acid bath 166 with capability to suspend the blocks 160,162and loop 164 in suitable position for acid 168, preferably sulfuricacid, to dissolve protective buffer coatings 100, 102 from the U-shapedloop 164. The length of coating 100, 102 removed from an optical fiber20 will depend upon the depth to which the optical fiber loop 164extends below the surface 170 of the acid.

[0122] Loops having substantially a desired size and shape formrelatively easily, but individual fibers need careful handling to avoiddamage to exposed glass surfaces. The “U” shaped loop 164 of fiber 20lies on one side of a pair of blocks 160,162 with relatively longtrailing fiber ends 172 extending from the opposite side of the blocks160,162. In this arrangement the loop 164 and the fiber ends 172 are atrisk for breakage or related damage. Damage occurs in different waysincluding inadvertent contact or impact during fiber processingoperations including acid removal of buffer coating 100, 102 from thefiber 20, fiber Bragg grating writing, fiber annealing, fiber recoatingand the like. This problem may now be avoided using filament organizers10 according to the present invention. Filament in the form of opticalfiber 20 is readily loaded onto filament organizers 10 without damage.Also the design of a filament organizer 10 allows stacking of multipleorganizers to increase process throughput. A stacked configuration 68 offilament organizers places filaments, i.e. optical fibers 20, in asuitably spaced-apart relationship for processing from fiber strippingto fiber Bragg grating recoating, as further described below.

[0123]FIG. 19 indicates arrangement of a filament organizer 10 for acidstripping using an apparatus that will reposition an optical fiber 20 ina filament organizer 10 to produce a suspended optical fiber loopsimilar to the previously described loop 164 formation between blocks160, 162. The apparatus grips the coated optical fiber 20 at two pointsalong its length as indicated in FIG. 19. A loop forms when these twopoints move toward each other, as shown in FIG. 20. The fiber is held atone point between a first pair of rolls 180 and at a second point by asecond pair of rolls 182. A layer of elastomer covers a lower roll 184,188 of each pair of rolls 180, 182. Each roll diameter is based on theminimum advisable fiber bend diameter to avoid inducing damaging bendingstress during optical fiber manipulation to form a loop. The elastomerprovides compliance to lower contact stress, reduce fiber slip, and holdmultiple fibers simultaneously.

[0124] Each pair of rolls 180,182 converges to pinch the fiber 20. Next,the upper roll 186, 190 of each pair rotates toward each other, inducinga shallow bend in the fiber 20. The shallow bend establishes a plane tobe occupied by a machine-formed loop when the pairs of rolls 180, 182move toward each other. The looping method works with the fiber tray 10by using two removable upper rolls 186, 190 and two non-removable lowerrolls 184, 188.

[0125] The present invention, in one of its embodiments, facilitates theprocess of acid stripping of multiple fibers in a single operation.Successful processing of multiple fibers requires several features thatare possible using filament organizers according to the presentinvention. Of particular importance is the use of a tool that shapesoptical fiber into loops while minimizing the possibility of fiberdamage. A suitable loop-shaping tool produces loops with repeatable sizeand shape. Once formed, loops preferably do not bend out of plane. Thislatter feature is important to the processing of multiple fibers thatwould tend to interfere with each other if out-of-plane bendingoccurred. Also, loops formed using stacked configurations of multiplefibers possess substantially the same size and shape, as required forautomated and/or semi-automated processing.

[0126]FIG. 21 shows a perspective view of a loop former 200 according tothe present invention including detail of the position of a stackedconfiguration 68 of filament organizers 10 inside a loop formingcontainer 202. The loop-forming container 202 is essentially a closedbox including a floor 204, a front wall (not shown), a rear wall 206 andfirst 208 and second 210 sidewalls. Inside the container 202, a firstledge 212 occupies a position adjacent the junction between the firstsidewall 208 and the floor 204. A second ledge 214 occupies a similarposition adjacent the junction of the second sidewall 210 and the floor204. The floor 204 includes a plurality of longitudinal slits 216disposed orthogonally towards the first 212 and second 214 ledges. Eachof the front wall and the rear wall 206 includes a generally U-shapedguide slot 218 that is indicated in dotted line form in FIG. 21.Preferably the guide slot 218 includes a horizontal slot 220 joined toan angled slot 222, at one end, and an opposed angled slot 224 at theother. A seated roller 226 includes an axle 228 engaging one end of thehorizontal slot 220 in the front and rear 206 walls of the loop-formingcontainer 202. The seated roller 226 preferably has a covering of anelastomeric material. A movable roller 230 includes axle ends 232extending into the horizontal slot 220 at the front and rear 206 walls.The movable roller 230 is repositionable along the length of thehorizontal slot 220 to facilitate formation of multiple extended loops234 from a stacked configuration 68 of filament organizers 10 positionedin the loop-forming container 202.

[0127] The loop former 200 provides accommodation for a stackedconfiguration 68 of filament organizers 10. Installation of a stackedconfiguration 68 inside a loop-forming container 202 requiresorientation of the stacked configuration 68 to provide alignment offilaments 20 with the plurality of slits 216 in the floor 204 of thecontainer 202. This may be accomplished by holding a stackedconfiguration 68 by the openings 26 in support boards 12 followed bylowering the stacked configuration 68 into the loop-forming container202 until the mounting plates 22 of the filament organizers 10 rest onthe first 212 and second 214 ledges, as shown in FIG. 21. The stackedconfiguration 68 is installed with the seated roller 226 and movableroller 230 at opposite ends of the horizontal slot 220. Correctpositioning of the stacked configuration 68 relative to the seated 226and movable 230 rollers provides gentle contact between each filament 20and the surfaces of the rollers 226, 230. In this condition, thefilaments 20 should still be under tension and free from bends.

[0128] Having positioned the stacked configuration 68 in theloop-forming container 202 with the filaments 20 touching the fullyseparated rollers 226, 230 a first rod 236 inserted through the angledslot 222, of the front wall, extends across the container 202 to enterthe angled slot 222 in the rear wall 206 of the loop forming container202. After insertion, the first rod 236 occupies a step 238 of theangled slot 222. A second rod 240 similarly positioned, in a notch 242of the opposed angled slot 224, completes formation of the first pair180 and second pair 182 of rolls. Movement of the first 236 and second240 rods to follow the contours of the angled slot 222 and the opposedangled slot 224 initially establishes contact between the rods 236, 240and the filaments 20. An elastomer band stretched over the ends of eachpair of rolls 180, 182 draws the rolls together to increase theirgripping force on the filaments 20. With continued gentle urging, therods 236,240 continue movement towards the horizontal slot 220. Thismovement places the rods 236,240 side by side with the correspondingrollers 230, 226 at each end of the horizontal slot 220. During thismovement the filaments 20 begin to wrap around the rollers 226, 230 inresponse to downward force applied by the rods 236,240. The appliedforce also draws filament from each rotary spool 18 with the resultingformation of preformed loops having a height approximately equal to thediameter of the rods 236, 240. Preformed loops become extended loops 234of greater height through movement of the first pair of rolls 180towards the second pair of rolls 182. This creates an exposed loop 234protruding through each slit 216 in the floor 204 of the loop-formingcontainer 202. By design, the slits 216 limit the amount of out-of-planebending by the filaments 20. Design features of each slit 216 include aloop entry 244 about 15.0 mm (0.625 inch) wide, and a narrower loopstation 246, having a width of about 1.6 mm (0.064 inch). The loop entry244 is wider than the loop station 246 to prevent contact of themechanically stripped fiber portion of an optical fiber 20 with thesides or other parts of a slit 216 during the initial stages of exposed,extended loop 234 formation. As the height of the loop 234 increases,the mechanically stripped portion of the fiber emerges below the floor204 of the loop-forming container 202. Any optical fiber 20 residing ina slit 216, at this point, has a covering of buffer coating to protectthe optical fiber from contact with any of the slit 216 surfaces. Thus,protected, the looped optical fiber 234 enters the narrow loop station246 where it will stay during removal of residual primary and secondarybuffer coatings by acid stripping. The loop entry 244 opens to the upperand lower surfaces of the floor 204 of the loop-forming container 202.Preferably the opening to the upper surface of the floor 204 is wider(15.0 mm) than the opening (12.5 mm) to the lower surface of the floor204. This description indicates that the loop entry 244 of each slit 216has a somewhat V-shaped cross section to assist in controlling thespatial positioning of each exposed, extended loop 234. Loop controlprovided by each narrow loop station 246 counteracts torsional stressesintroduced during manufacture of filaments 20 in the form of opticalfibers. It will be readily appreciated that each extended loop 234 hangsbelow the floor 204 of the loop-forming container 202 ready forimmersion in an acid-containing bath 166 as indicated in FIG. 20.

[0129]FIG. 22 shows the condition of an optical fiber 20 after theprocess steps of mechanical stripping, and acid stripping to provide alength of an optical fiber 20 having a buffer-free bare central portion250 suitably prepared for refractive index modification associated withthe writing of a Bragg grating. Before the actual writing of a gratingoccurs, the unspooling of an end section of fiber provides an opticalfiber pigtail 252 suitable for the formation of splices or connectionsbetween optical fibers 20. At this point in the process, a pigtail 252at each end of the optical fiber, carried on the filament organizer 10,provides a point of connection so that the progress and accuracy ofgrating writing may be optically monitored during the writing process.

[0130] Optical fiber Bragg gratings may be written in a plurality ofoptical fibers 20 each having a bare central portion 250. Convenienthandling of these fibers 20 uses filament organizers 10 in a stackedconfiguration 68 according to the present invention. The pigtails 252 ofeach optical fiber 20 require positioning using fixtures to permitaccurate alignment of fiber ends with equipment that monitors theprogress of fiber Bragg grating writing. The fixture is an optical fiberconnector including a central body with opposing fiber receiving ends.The monitoring equipment may use optical fiber or non-contact couplingto the pigtail ends to complete the optical circuit. Connection tofibers 20 from each filament organizer 10 in a stacked configuration 68uses pigtail ends adapted to plug into the connector. A connector mayaccommodate pigtail ends of a single optical fiber 20 or a plurality offibers 20 having pigtail ends previously terminated to the requirementsof a multi-fiber connector. The term “ribbonized” has been applied toone form of termination wherein the ends of pigtail sections of fiberlie side by side to form a single layer of fibers having a flatribbon-like appearance. Positioning of the pigtail ends allows them tomate with a multi-fiber connector.

[0131]FIG. 23 indicates a filament tensioning apparatus 260 used forfurther processing of an optical fiber 20 held in a filament organizer10 according to the present invention. After placement of an opticalfiber 20 on a support board 12, as discussed previously, the portionlocated between the mounting plates 22 of the filament organizer 10exists in a condition of tension applied by a tensioner 58. Inpreparation for the writing of a grating, the tension wire 62, of aselected filament organizer 10 (see FIG. 3), may be grasped in thevicinity of the notch 66 and extended slightly parallel to the edge ofthe board 12, and toward the nearest guide 24. Movement of about 1.0 mmto about 2.0 mm releases the tension force applied to a filament 20 bythe tensioner 58. Preferably the filament organizer 10 is one of astacked configuration 68 positioned on a platform of an indexing unit.The indexing unit raises and lowers the stacked configuration 68 usingany one of a variety of mechanical and hydraulic structures. In oneembodiment, sliding engagement of the platform with one or more verticalposts or beams allows upward and downward movement of the platform andstacked configuration it supports. The platform may include organizingmounts for mating engagement with spacer blocks on the lowest filamentorganizer so that the stacked configuration 68 is suitably aligned withthe optical fiber 20, of each filament organizer, accessible to a fibertensioning apparatus 260.

[0132] The indexing unit remains stationary during the approach of afiber tensioning apparatus 260 to apply clamps and grippers to a centralportion of each optical fiber 20 in a stacked configuration 68. Approachof the fiber tensioning apparatus 260 may be facilitated by an alignmentmechanism for optimum positioning between an optical fiber 20 and clamps262, 264 of a fiber tensioning apparatus. Optimum alignment does notnecessarily require attachment of the indexing unit to a fibertensioning apparatus 260.

[0133] Clamps 262, 264 attached at each end of the central load-freefiber portion 20 retain the central portion in its load-free condition,before subjecting the optical fiber to a selected tension force. Theclamps comprise components of a fiber Bragg grating tensioning holder266 used to stretch the fiber 20 under a prescribed load during fiberBragg grating writing. A tensioning holder 266 according to the presentinvention comprises a voice coil 268 as a load applicator to a load cell270 that measures the load applied between the pair of clamps 262, 264holding the central portion of the fiber 20. After precise applicationof a desired selected tension, a pair of grippers 272, 274 isolates ameasured fiber portion 276, between the clamps 262, 264, setting up atension zone independent of outside tension variations. This maintains aprescribed load on the measured fiber portion 276 and prevents any fiberslippage relative to the grippers 272, 274 and hence the clamps 262,264.The fiber Bragg grating may be written into the bare portion 250, of theisolated, measured fiber portion 276 held between the pair of grippers272, 274. Tension applied to a clamped optical fiber 20 anticipatesshrinkage that will occur, changing the separation between gratingfeatures after a grating has been written and after a piece of axiallystrained measured fiber portion 276 has been released from the pair ofgrippers 272,274.

[0134] A voice coil driven tensioning holder 266 is favored over any ofseveral possible load applying units including a DC servo motor andencoder combination, a precision pneumatic cylinder, a high precisionmicro-positioning linear stepper motor and a mechanical balance beam. Aprecision pneumatic cylinder, for example, provides insufficient fibertension and fine pressure control to accurately apply a prescribedamount of tension to an optical fiber. A high precisionmicro-positioning linear stepper motor is equally incapable of providingrequired precise tension adjustment. Problems associated with the use ofa mechanical balance beam include the fact that it is primarily a manualprocess not particularly conducive to automation.

[0135] Voice coil activated clamping structures are known. For example,U.S. Pat. No. 4,653,681 describes a voice coil activated fine wireclamp, used in wire bonding applications. Clamp jaws may be moved to anopen position from a normally closed position using a voice coil motorunder microprocessor control. A voice coil programmable wire tensioner,described in U.S. Pat. No. 5,114,066 also facilitates wire bonding. Thisshows that it's known to use a voice coil in wire bonding applications.However, it appears that the use of a computer controlled, voice coilmotor has not been used to apply repeatable, precise amounts of tensionto optical fibers for consistent production of optical fiber Bragggratings having essentially the same wavelength response.

[0136] The advantageous use of a voice coil actuator 268 provides alinear output force corresponding to an input current that may be finelycontrolled. A high precision power supply with a voice coil actuator 268produces a stable signal leading to an output force that is remarkablyconstant. This allows selection of a wide range of output force, limitedonly by the magnitude of energy transfer between a coil and a magnet.The output force of the actuator 268 is proportional to the inputcurrent, similar to a DC motor. A tensioning method based upon a voicecoil actuator 268 occurs in response to bearing-free passage of energybetween a coil and a magnet. Tension adjustment using this method offerssignificant advantages over prior methods that used addition and removalof static weights to increase or decrease tension on a fiber.

[0137]FIG. 23 shows that the mounts 278, 280 for the voice coil and loadcell include air bushing carriages 282, 284 for minimal frictionrelative to a support bar 286. Air bushing carriages 282, 284 reducestatic friction in the system to a low, almost insignificant level.Reduction of friction in the bushings 282, 284 allows accurateapplication of fiber tension corresponding to the force acting on theload cell 270. This results in improved control of the force generatedby the voice coil actuator 268 and more consistent application oftension to an optical fiber 20. Each carriage 282, 284 includes a clamp262, 264 for attachment to a central portion of an optical fiber 20. Theseparation between the clamps 262, 264 identifies the central portion ofthe optical fiber 20 to be tensioned. An extending guide rod 288attached to the moving coil of the voice coil actuator 268 pushesagainst the load cell 270 increasing separation between the twocarriages 282, 284. Increasing separation between the carriages 282, 284operates through the clamps 262,264 to move them away from each other toadd strain to the optical fiber 20. Upon attainment of a desired straina pair of grippers 272, 274 grasp an inner measured fiber portion 276 ofthe optical fiber 20. The measured fiber portion 276 is somewhat shorterthan the central portion between the clamps 262, 264. Accuratemaintenance of force at selected levels allows the writing of acceptablefiber Bragg gratings. Force selection and control relates to the use ofa high precision load cell 270 to measure and display the tensioninitially applied to the fiber 20 and maintained during the writingprocess. The load cell 270 may also provide feedback during computercontrolled automated fiber Bragg grating writing.

[0138] An important aspect of writing a fiber grating is the need tohold a measured fiber portion 276 in a fixed, immobile conditionthroughout the process. This requires the use of a jaw assembly 290attached particularly to the grippers 272, 274 for removably securing ameasured fiber portion 276 in the desired immobilized condition. Clamps262, 264 attached to the filament tensioning apparatus 260 may use thesame jaw assembly 290 or another providing adequate clamping of acentral portion of a fiber 20.

[0139]FIG. 24 illustrates a fiber portion gripper 272 with an attachedjaw assembly 290 of suitable design. The jaw assembly 290 comprises alower jaw 292 attached at the end of a gripper 272 and an upper jaw 294for engagement with the lower jaw 292 to grip and immobilize a measuredfiber portion 276, shown in cross section in FIG. 24.

[0140]FIG. 25 provides a detail drawing of a V-shaped channel 296 formedin the upper surface of the lower jaw 292 and a rectangular crosssection groove 298 in the lower surface of the upper jaw 294. The sizingof each of the channel 296 and groove 298 of the jaw assembly 290corresponds to the diameter of the measured fiber portion 276 that isheld in an immobilized condition.

[0141] A jaw assembly 290 design requires matching of the dimensions ofa fiber 20 with those of a V-shaped channel 296 and a rectangular groove298. Dimensional matching allows the application of substantially equalpressure at contact points around the circumference of a measured fiberportion 276 held immobile for the writing of a Bragg grating accordingto the present invention. Preferably a fiber 20 is held between theV-shaped channel 296 and the rectangular groove 298 with equal pressureapplied at points of contact around its circumference. This is indicatedin FIG. 25 by the fact that the two points of contact of the fiber 20with the V-shaped channel 296 and the fiber's point of contact with thegroove 298 are equidistant from the bare optical fiber 106. Uniformapplication of pressure leads to even distribution of stresses acrossthe diameter of a measured fiber portion 276 to reduce fiber damage to aminimum, considering the amount of pressure required for the grippers272, 274 to hold the measured fiber portion 276 in an immnobilecondition. V-groove chucks are known for clamping portions of opticalfibers, as taught by U.S. Pat. Nos. 4,623,156 and 5,340,371. It does notappear in either case that consideration is given to equalizing theamount of pressure applied to points around the circumference of afiber.

[0142] In a preferred embodiment of the present invention, pressureequalization around the circumference of an optical fiber 20 requiresthe use of a floating upper jaw assembly 295, as shown in FIG. 26. Theself-adjusting, floating upper jaw assembly 295 comprises a fiber clasp300, a support flange 302, and an angular compensator 304 (see FIG. 27)separating the fiber clasp 300 from the support flange 302. A fiberclasp 300 may also be referred to herein as a filament clasp. Eachgripper 272, 274 includes, in this case, an upper jaw mount 306 and alower jaw mount 308. A lower jaw 292 attaches to the lower jaw mount 308and an upper jaw assembly 295 attaches to the upper jaw mount 306 by thesupport flange 302. Suspension of a fiber clasp 300 from a supportflange 302 preferably uses a spring-loaded connector 310. Spring tensionoperating between the fiber clasp 300 and support flange 302 retains anangular compensator 304 between them. During capture of a filament 20between the lower jaw 292 and upper jaw 294 of a gripper 272, 274, theuse of a floating upper jaw assembly 295 allows application of grippingforce to a filament 20 substantially without displacement or rotation ofthe filament 20. The clamps 262, 264 may also include a floating jawassembly.

[0143]FIG. 27 shows the result of gripping a filament 20 between afloating jaw assembly 295 and a lower jaw 292. As the floating jawassembly 295 moves towards the lower jaw 292, the rectangular groove 298of the filament gripper 292, 294 makes contact with a filamentpositioned in the V-shaped channel 296 of the lower jaw 292. As contactoccurs, the filament clasp 300 may adjust slightly to apply grippingforce uniformly to the filament 20, without disturbing it. Adjustment ofthe filament clasp 300 relies upon its independent movement due to theangular compensator 304 that separates it from the support flange 302. Apreferred angular compensator 304 according to the present inventioncomprises a spherical element that prevents contact between the filamentclasp 300 and the support flange 302. Preferably the angular compensator304 seats between a substantially conical shaped depressed portion 301in the fiber clasp 300 and a substantially conical recess 303 in thesupport flange 302. The angular compensator 304 maintains separation ofthe support flange 302 from the filament clasp 300 to allow them to moveindependently. Also, the spherical structure of the angular compensator304 allows effective change of angle around the perimeter of thefilament clasp 300.

[0144] The previous discussion provided a description of positioning,clamping and gripping a single optical fiber 20 using an apparatus 260including a tensioning holder 266 to tension the fiber 20 during writingof a Bragg grating. The description involves the relative positioningbetween a filament organizer 10 and a tensioning holder 266. When afilament organizer 10 represents one of a number of organizers 10 in astacked configuration 68 the writing of a Bragg grating may beaccomplished in a variety of ways. For example, fiber optic Bragggratings may be written one at a time using a step and repeat process tomove a fiber 20 carried in a selected filament organizer 10 into thecorrect position, relative to the tensioning holder 266 to executewriting of a Bragg grating. The wavelength response of an optical fiber20 may be monitored, as described previously, during Bragg gratingwriting. An alternative to sequential writing of Bragg gratings may beto use a bank of tensioning holders 266 and related writing devices forproducing a plurality of Bragg gratings simultaneously.

[0145] The step and repeat process using an indexer to reposition astacked configuration 68, e.g. preferably by up or down directionalmovement, presents a new fiber to the Bragg grating writing device. Thestacked configuration 68 fits into the platform of an indexer adapted toprovide mating with a known positional relationship between a stackedconfiguration 68 and the platform using alignment between the spacerblocks 70 and organizer mounts 72. Having established the preferredplacement of the stacked configuration 68 relative to the indexer, andhaving made connection of the fiber pigtails 252 to the opticaldetection system, a scan of each fiber verifies the existence ofreliable optical connections.

[0146] The placement of a stacked configuration 68 in an indexer withfiber optic connection to an optical detection system precedes seriateBragg grating writing process in which the indexer initially uses anoptical sensor to scan the filament organizers 10, counting the numberin the stacked configuration 68. This process designates the firstfilament organizer 10 in the stacked configuration 68. A sequence ofoperations modifies the optical fiber 20 held in this first filamentorganizer 10. Before modifying the optical fiber itself, removal of theeffect of the constant force tensioner 58, as described previously, usesa force reduction assembly comprising a pair of pulleys 64 on eitherside of a notch 66 formed in an edge of the support board 12. Thetension wire 62, passing around the pulleys 64 and across the notch 66,may be grasped in the vicinity of the notch 66 and extended slightly,parallel to the edge of the board 12, and toward the nearest guide 24.This releases the tension on the rotary spool 18 of the filamentorganizer 10, thereby releasing the tension in the filament or opticalfiber 20.

[0147] Preparation for modifying a filament 20, in the form of anoptical fiber, requires securing the tension-free portion of the opticalfiber using an apparatus that combines a filament tensioning apparatus260 and an interference pattern generator (not shown). Each of thefilament tensioning apparatus 260 and the interference pattern generatormay be moved separately initially to secure and position an opticalfiber 20, as discussed previously, and then to modify the fiber'sstructure.

[0148] The filament tensioning apparatus 260 grips the fiber 20, asdescribed above, using a first clamp 262 and second clamp 264. Forceoperating between the two clamps 262, 264 applies tension to the portionof optical fiber 20 between them. The force may be generated using avoice coil actuator 268. The amount of tension is predetermined andmeasured using a load cell 270. At this point the optical detectionsystem provides a reference scan of the optical fiber 20, including theportion held under tension between the clamps 262, 264.

[0149] To reproducibly modify an optical fiber 20, preferably a measuredportion 276 of the fiber 20 remains in a fixed condition held by a firstgripper 272 and second gripper 274 that grip the fiber and hold it. Oncethe measured portion 276 of the optical fiber has been immobilized, aninterference pattern generator moves into close proximity to themeasured portion 276 of the optical fiber 20. Light, from a containedlaser source, passes through an opened shutter, and an optical system,including the interference pattern generator to produce an interferencepattern. The proximity of the interference pattern generator to theoptical fiber 20 provides sufficient energy to reproduce the linecharacteristics of the interference pattern or interferogram in the core106 of the optical fiber 20, preferably within the measured fiberportion 276. Impingement of actinic radiation, produced by anultraviolet laser, produces an optical fiber Bragg grating as a resultof changes in refractive index in parts of the optical fiber core 106affected by the radiation. Refractive index modulation corresponds tothe pattern of the interferogram, produced by the interference patterngenerator. Progress in reproduction of an interferogram in the core ofan optical fiber may be monitored using an optical detection system fordata acquisition. Data acquisition follows changes in the transmissionspectrum produced by a developing Bragg grating with time. Upon sensingthe desired transmission spectrum, the optical detection system closesthe shutter to prevent further exposure of the optical fiber to laserlight.

[0150] Following completion of optical fiber modification and removal ofthe interference pattern generator from the vicinity of the measuredfiber portion, the grippers 272, 274 and clamps 262, 264 retract fromthe fiber 20 to allow the filament tensioning apparatus to move to thenext filament organizer 10 in the stacked configuration 68. Onceseparation of filament organizer 10 from the Bragg grating writingequipment occurs, the force reduction assembly releases the opticalfiber placing it once again under the tension generated by the tensioner58 of the filament organizer 10. This completes the modification of agiven optical fiber so that the indexer can readjust to align theoptical fiber 20 in the next filament organizer 10, in a stackedconfiguration 68, with the filament tensioning apparatus and theinterference pattern generator before repetition of the Bragg gratingwriting cycle.

[0151] Annealing using an annealing oven at 300° C. for 10 minutesprovides stabilization for a Bragg grating produced by refractive indexalteration of an optical fiber. An annealed Bragg grating may requireprotection by recoating the central portion of the optical fiber, whichwas previously stripped of protective coating. Any of a number ofmethods may be used for protective recoating of optical fiber Bragggratings including in-mold application, extrusion coating and spraycoating a fiber with a curable liquid coating. Equipment is commerciallyavailable for in-mold application of liquid recoat formulations. Thequality of in-mold optical fiber section recoating varies with the skillof an operator to carefully position a fiber in a mold cavity. Also,product yields have been reduced because of coating defects and fiberstrength issues associated with fiber handling and sectional recoating.As alternatives, either spray coating or extrusion coating may be usedfor recoating optical fibers that include Bragg gratings according tothe present invention.

[0152] A filament organizer 10 according to the present invention may beused to advantage for positioning uncoated portions 250 of and opticalfiber 20 in a fiber recoating mold. Since the filament organizer 10 alsoapplies tension to the optical fiber 20, an alignment plate attached toa mold recoater of the type supplied by Vytran Corporation ofMorganville, N.J. is the only requirement for correct fiber positioningwithin a groove such as a semicircular or V-groove of the split moldapparatus. The alignment plate may use strategically positioned studs toengage the through holes 80 of the planar support 12 of a filamentorganizer 10. This eliminates the need for mold positioning usingmicromanipulator platforms and the like. The effective diameter of thegroove is somewhat greater than that of the remaining coated portions ofthe fiber. Due to pre-tensioning of the fiber by the filament organizerthe common need for external tensioning is eliminated. Once thevulnerable uncoated portions 250 of the fiber 20 have been suspendedclear of the groove surface, the hinged mold is closed and recoatingmaterial is injected into the groove until it extends to the coatedportion of the fiber. The molding material is then cured yielding arecoated section with dimensional characteristics essentially identicalto those of the original coated fiber.

[0153] Fiber recoaters of the type described briefly above include asplit steel mold, each portion of which contains a matching semicirculargroove to accommodate the fiber. The grooves, when clamped together,formed a cylindrical bore slightly larger than the coated fiber OD topermit escape of air during injection of the coating material. Theoriginal coating in this arrangement keeps the uncoated sectionsuspended in the bore. A short uncoated length of fiber, typically nomore than half an inch, minimizes the possibility of damage throughcontact with the bore. Also, a series of clamps, attached on either sideof a central fiber portion, prevent the uncoated portion from touchingthe bore. Before injecting recoating fluid, the upper half of the moldis clamped in position to form the cylindrical bore. The curablerecoating fluid may be a room temperature curing epoxy resin or otherresin that cures either at elevated temperature or in response tosuitable radiant energy such as ultraviolet radiation.

[0154]FIG. 28 illustrates the use of a filament organizer 10 to store anoptical fiber 20 having a bare portion 250 that has been modified toinclude a Bragg grating. An exposed Bragg grating may be recoated afterpositioning the filament organizer 10 in a suitable spray recoatingapparatus 320. For correct positioning of a filament organizer relativeto a spray recoating apparatus 320 the bare portion of an optical fiber250 lies in the path of spray ejected from a recoating spray head 322.Such correct positioning is achievable by any of a variety of methodsand devices. One such method uses a plate suitably positioned relativeto a spray recoating apparatus and including alignment studs to engagethrough holes in a filament organizer 10 to place the bare portion 250of an optical fiber 20 in the optimum position for application ofrecoating spray.

[0155] A spray recoating apparatus 320 comprises at least one recoatingspray head 322 and a radiation source 324. A filament organizer 10 isadapted for oscillatory movement of the bare portion 250 of an opticalfiber between the recoating spray head 322 and the radiation source 324.Preferably, the position of the recoating spray head 322 is from about 1cm to about 2 cm from the fiber 20, preventing contact between the sprayhead 322 and a deposited coating. The spray recoating method providescontrolled sectional recoat that achieves performance characteristicsnot obtainable from conventional in-mold recoating processes. It is anon-contact method since the optical fiber 20, including the bareportion 250, does not touch any part of the recoating equipment. Thisrepresents another benefit of suspending a fiber 20 in a filamentorganizer 10 that may be readily attached to the recoating apparatuswith precise fiber 20 to spray head 322 alignment. Another benefit ofspray recoating involves over coating one recoating composition withanother exhibiting different properties to produce a multilayer bufferstructure, around a fiber, including layers that differ in propertiessuch as modulus and durability or hardness.

[0156] The use of a spray recoating process allows flexible placement ofa single filament or multiple filaments in the path of spray or mistfrom a recoating spray head 322. Where a filament organizer 10 providesthe preferred means for handling an optical fiber 20, several filamentorganizers 10 may be closely located with variable orientation to placea plurality of fibers in the path of a single spray or directed mist. Anadditional advantage of spray recoating versus conventional cavity-moldrecoating is the provision of a recoating spray head 322 that may beadjusted or translated to differing lengths of bared optical fiberportions 250.

[0157] As the bared portion 250 of a fiber 20 traverses the location ofthe recoating spray head 322, one side of the bare fiber portion 250receives a light deposit of droplets from a mist of a curable recoatingcomposition. Movement of the filament organizer then places the depositof droplets in the illumination path of the radiation source 324. Theradiation cures the layer of recoating composition. Returning to thelocation of the recoating spray head 322, the filament organizer 10flips over to expose the opposite side of the previously bare fiberportion 250 to the spray of curable recoating composition. This allowsapplication of a fine mist of recoating composition to the exposedoptical fiber surface. This layer may be cured as described previously.Repeated processing by coating and curing with oscillation and flippingof the filament organizer 10 protects the fiber with multiple layers ofrecoating composition. The recoated fiber surface has a matte appearanceresulting from the build up of successive layers of coating material.Coating topography up to about 15 μm was revealed on the surface of amicroscope slide by surface scanning with an ELFA STEP mechanical stylusprofilometer available from Tencor Corporation.

[0158] Approximately fifty applications of recoating compositionfollowed by curing, after each pass, provide a layer having a thicknessover the recoated length similar to that of the original buffer coatingson other parts of an optical fiber 20. This technique allows layers ofrecoating composition to be applied to the surface of an optical fiberto build a protective recoat having a thickness of from about 10 micronsto about 100 microns on a bare fiber 106. The diameters ofspray-recoated optical fibers may be measured using a microscope and aQUADRA-CHEK 2000, from Metronics Inc., Bedford, N.H. Coating thicknessmay be varied depending on the application.

[0159] Another embodiment of the present invention provides a secondrecoating spray head 326 and optionally a second radiation source 328positioned opposite the previously discussed recoating spray head 322and radiation source 324. The description of multiple spray heads 326and radiation sources 328 as occupying opposing or staggered opposingpositions includes alignment of positions but is not limited thereto.Any number of spray heads, positioned strategically, may be used in afiber recoating process. Placement of a spray head and radiation sourceon both sides of an optical fiber 20 facilitates recoating of both sidesof the bare fiber portion 250, while eliminating the need to flip thefilament organizer through 180°. As indicated previously, the use ofadditional radiation sources 328 is optional since the beam from asingle radiation source 328 may be directed to effect curing around thecircumference of a recoated fiber.

[0160] The contours of a deposit of droplets applied to a bare fiber 106will reflect the size and shape of the droplet cloud issuing from aspray head 322. If required, a means for shaping the droplet cloud couldproduce a desired pattern of droplets on the surface of a fiber 20.Suitable shaping means include stencils, other types of masking devices,and stream deflectors such as air knives.

[0161] A preferred recoating process according to the present inventionuses an air knife to direct an atomized stream at various angles ofcontact with an optical fiber 20. Air knife adjustment of the shape of adroplet cloud, and its angle of impingement with an optical fiber 20,may allow the use of a minimum of spray heads 322, 326 to achieveoptimum fiber recoat uniformity and concentricity. Also, the use of airknife deflection of small volumes of recoating compositions provides anadvantage when compared to the control of diverging streams ofrelatively high volume spray heads described in Japanese patent JP60-122754. U.S. Pat. No. 5,219,120 teaches the use of an air horn thatprovides a moving sheet of air to entrain a substantially uniform lineardispersion of atomized fluid moving above and extending substantiallyacross the width of the air horn. The air horn spreads the dispersion ofatomized fluid to a width suitable for spraying the flat surface of acircuit board. Such extensive spreading of a cloud of droplets does notapply directly to narrow curved surfaces such as those of an opticalfiber. Also, the air horn described in U.S. Pat. No. 5,219,120 is aseparate structure from the fluid atomizer.

[0162] Preferably air knife deflection according to the presentinvention occurs through the use of an air knife attachment that fitsover the exit nozzle of a spray head. The air knife attachment includesa pair of receiving chambers, at least one on either side of the sprayhead, into which air may be directed. Each receiving chamber has an airentry at one end connected to an air reservoir. The opposite end of eachchamber includes an air knife slit that exits from the chamber at anangle to the axis of the spray head. Air issuing from an air knife slitdeflects the spray cloud, generated e.g. by an ultrasonic atomizingspray head, at an angle corresponding to the angle formed between theslit and the axis of the spray head. Independent operation of each airknife, described above, causes selective deflection of a spray cloud atan angle that directs the droplet cloud towards an uncoated portion ofan optical fiber. Selective deflection of a droplet cloud allowspositioning of a number of optical fibers around a spray head nozzle.Impingement of air from exit slots of air receiving chambers deflectsatomized spray at various angles for sequential recoating of the numberof optical fibers held around the spray head using filament organizers10 according to the present invention. The use of air deflectionpreferably requires that the recoating composition is not oxygeninhibited. This does not prevent the use of oxygen inhibited recoatingfluids providing an inert gas is connected to the receiving chambers ofthe air knife attachment.

[0163] The process of recoating a bared portion 250 of an optical fiber20 may use spray heads 322, 326 based upon either ink jet or ultrasonicatomization technology. Preferably, the application of curable recoatingcomposition, to an optical fiber 20, uses ultrasonic atomizationtechnology to dispense small particles (<50 μm) of a fluid, having aviscosity from about 40 to about 900 centipoises, preferably 40centipoises to about 400 centipoises, over a bared portion 250 of thefiber 20. Viscosity measurements were made at 25° C., using a BOHLINCS-50 rheometer. Other requirements for a coating composition forrecoating optical fibers according to the present invention depend uponthe intended use of a recoated optical fiber device, such as a Bragggrating. Example 1 of Table 1 provides a load. bearing coating,preferably having a high modulus, high glass transition temperature(Tg), and temperature stability above the upper operating temperaturefor a selected application. Examples 2 and 3 produce cured coatings thatflex and bend with a recoated portion of a fiber. Preferably coatingcompositions, in this case, possess thermomechanical properties similarto undisturbed buffer coating, originally applied to the fiber.Immediate curing of such a coating reduces undesirable agglomeration,which could result in beading or poor concentricity.

[0164] An ultrasonic atomization processes differs from a sprayatomization process that, requires air velocity to break up a sprayablecomposition into droplets. Droplet size of a spray atomization processis larger (50 to 100 microns diameter) and the spray velocity, at itslowest pressure of ˜20 psi, propels the droplets with a force causingthe droplets to spread upon impact with a fiber surface. Beingrelatively high, the impact force of an air atomized spray against afiber causes build-up of agglomerated droplet beads accompanied byformation of a non-concentric coating.

[0165] The ultrasonic atomization process generates volumes of coatingcomposition that are extremely small, in the range from about 0.001ml/min to about 0.010 ml/min using a 2.0 cc glass syringe available fromPopper & Sons. The flow rate for dispensing a substantiallynon-directional cloud of droplets less than 50 microns in diameterdepends upon the speed at which the fiber is scanned in front of theatomizer head. A low velocity flow of nitrogen, or other inert carryinggas directs the cloud of ultrafine droplets of recoating compositiontowards a target surface. The low cloud volume and extremely smalldroplet size cause the formation of a textured discontinuous covering ofthe fiber surface. Although coatings are low enough in viscosity forspray application, preferred coating compositions exhibit minimal flow,after application, prior to coating. Flow and droplet agglomeration isfurther limited because the recoating composition, immediately afterapplication, undergoes exposure to curing radiation from the radiationsource 324, 328. Repeated application of recoating composition builds upa protective coating over a bared optical fiber portion 250. A recoatedoptical fiber preferably has a relatively smooth appearance bubble-freeappearance. This requirement guides the selection of materials used toprepare recoating compositions according to the present invention.

[0166] Suitable recoating compositions include low molecular weight, lowviscosity epoxy functional, 100% solids resins that photocrosslinkpreferably via an ionic mechanism initiated by a cationicphotoinitiator, especially an iodonium salt photoinitiator. Suchcoatings have good adhesion to the unstripped buffer coats on a fiber aswell as to the bare surface of the fiber. Ionic curing occurs withoutexclusion of oxygen. Radical curing recoating compositions may also beused in an inert environment. Suitable radiation sources forphotocrosslinking include those having wavelength emission in theblue/visible and ultraviolet wavelength regions of the spectrum. Curedcoatings according to the present invention.

[0167] A typical cured recoating composition has an elongation at leastequal to and preferably greater than that of glass, i.e. more than 7%.Also, a cured recoating composition has toughness and sufficientadhesion to glass to withstand accidental rubbing or contact with otherobjects during handling of a recoated fiber. TABLE 1 Filament CoatingFormulations Material* Example 1 Example 2 Example 3 Weight % Epoxy A57.0 — — Weight % Epoxy B 38.0 — — Weight % Epoxy C — 67.0 66.5 Weight %Epoxy D — 25.1 28.5 Weight % Polyether Glycol —  2.9 — Weight % IodoniumSalt  5.0  5.0  5.0 Solution

[0168] Measurement of Coating Composition Viscosity

[0169] A Bohlin Model CS-50 controlled stress rheometer was used tomeasure the viscosities of coating compositions, for recoating filamentsaccording to the present invention. The test method uses parallel plategeometry and “stress viscometry” mode. Viscosity measurement begins withplacement of a coating composition on the base surface of the parallelplate geometry. A second surface, mounted to rotate on a spindle, islowered into contact with the coating composition until a specified gapexists between the surfaces of the parallel plate geometry. Rotation ofthe spindle raises the rate of rotation to a number of revolutions perminute to produce a predefined stress (torque). The calculation ofviscosity values includes consideration of the geometry of the surfaces,the torque and the gap. Viscosities cited herein were obtained at 25° C.using a surface diameter of 20 mm, a gap between surfaces of 0.3 mm, anda stress of 93.8 Pascals.

[0170] A spray head that included an ultrasonic atomizer was used toapply curable recoating formulations, shown in Table 1, to the baresurfaces of several samples of silica fiber, each having a diameter ofabout 125 microns. Each formulation was dispensed via the tip of theatomizing horn of an ultrasonic atomizer available from Sono-Tek. Thepower supply of the ultrasonic atomizer was set to a level of 5.4 watts.Successful atomization of recoating formulations, having viscosities inthe range from about 40 centipoises to about 400 centipoises wasachieved using a micro-bore fluid delivery tube through the center ofthe nozzle body of the ultrasonic atomizer. Most preferably the coatingcomposition has a viscosity of about 200 centipoises. Recoatingformulations were supplied to the micro-bore tube at a syringe pumpdelivery rate of 0.015 ml/min. A preferred method uses a 21.5 gaugemicro-bore tube available from Small Parts Inc., Miami, Fla. Thisprovides precise control of small volumes of recoating compositiondelivered to the point of atomization.

[0171] Ultrasonic atomization as described previously produces anon-directional mist of coating composition that needs to be entrainedin a directional gas stream. Preferably the directional gas streamcomprises an inert gas, e.g. nitrogen gas, under the control of a shroudaround the micro-bore tube. A nitrogen gas stream flowing through theshroud around the atomizer head at a rate of 1.0 liter/min yields asuitably controlled atomized mist of recoating formulation. Adjustmentof the air shroud alters the contours of the gas stream therebymodifying the size, shape and coverage of a stream of droplets ofcurable recoating formulation impinging on a selected surface. Acontinuous coating may be formed on a surface using as few as about 4 toabout 6 applications of a coating formulation. However, depending uponprocess conditions, application of coating formulation may need to berepeated form about 40 to about 60 times to build a coating thickness ofup to 250 microns on a selected surface.

[0172] A filament recoating formulation was shown to produce a suitablestream of material for application using an ink jet printing/spray headas follows:

EXAMPLE 4

[0173] Epoxy A 76.0 weight % Epoxy B 19.0 weight % Photoinitiatorsolution  5.0 weight %

[0174] The photoinitiator solution contains 40 parts or iodoniummethide, 60 parts of decyl alcohol and 4 parts of isopropylthioxanthone.

[0175] The ink jet printing/spray head operated at a head temperature of70° C. A ink jet printing/spray head, available from TridentInternational Inc., Brookfield, Conn. was selected to apply recoatingcomposition to several samples of silica fiber, each having a diameterof 125 microns. The print head has 64 nozzles , each 50 microns indiameter. Use of a filament organizer mounted on a filament recoatingapparatus provided suitable alignment of a fiber with an ink jetprinting/spray head prior to application of recoating composition.Particles of the composition were jetted over a 1 cm length, on one sideof a filament of each of five samples of silica fiber. An EFOS ULTRACUREradiation source (EFOS Inc., of Mississauga, Ontario, Canada), with anultraviolet radiation wand, was used to direct energy to the coatedsample to initiate cure. Repeated passes under the recoating spray head,followed by ultraviolet radiation curing, produced adequate coverage ofthe bare optical fiber.

[0176]FIG. 29 provides a diagrammatic representation of a preferredsplit die extrusion coating method that uses an recoating fluidextrusion apparatus 330 having a die head assembly 332 encircling anoptical fiber 20 to recoat a bared optical fiber portion 250 thatcontains a Bragg grating. A study, reported in Electronics Letters Vol.34, No. 12, Jun. 11, 1998, investigated a split die recoating process toapply a solution of polyimide to a bare portion of an optical fiber. Theprocess involved drawing a fiber through the fluid filled split die,then driving off solvent at 70° C. followed by baking the polyimiderecoated section at 300° C.

[0177] Split die extrusion coating according to the present inventionoffers improvements for fiber recoating including controlled applicationand relatively low temperature curing of recoating compositions asfollows. The die head assembly mentioned above comprises a split sizingdie 334 and an in-line radiation cure chamber 336 that is closed aroundthe optical fiber 20. Accurate fiber 20 positioning, for recoating andprotection of the Bragg grating occurs during engagement of a filamentorganizer 10 with the recoating fluid extrusion apparatus 330. Any oneof a variety of methods may be used for engagement between a filamentorganizer 10 and an extrusion apparatus 330 provided that the die headassembly 332 has movable alignment to deposit a substantially uniformlayer around the fiber portion 250 that needs recoating. Duringrecoating, the bared fiber portion 250 of an optical fiber 20 remainsstationary between fiber positioners. The split sizing die 334 liesadjacent to one end of the bare fiber portion 250 from which curablerecoating composition will be applied across the remainder of the bareportion 250. Photocurable coatings extrude from the leading edge of thesizing die 334 as it traverses the length of the bared optical fiberportion 250. The radiation cure chamber 336 moves with the sizing die334 following behind it to initiate curing of the photocurable recoatingcomposition immediately after its deposition on the surface of theoptical fiber 20. The recoating composition curing reaction preferablyrequires an inert atmosphere. For this purpose an inert gas deliverytube 338 directs a flow of nitrogen into the radiation cure chamber 336that is illuminated using a suitable source of radiation, preferablyultraviolet radiation.

[0178] A linear transport mechanism 350 adjacent to the coating head 332includes a guide rod 352 and a carriage 354 slidably mounted on theguide rod 352 for movement along the guide rod 352. A connecting rod 356from the carriage 354 to the coating head 332 provides lineardisplacement of the coating head assembly 332 during movement of thecarriage 354 to move the coating head 332 from the first boundary to thesecond boundary of a bare portion 250 of an optical fiber 20. Curablefluid may be extruded from the sizing die 334 and energy from theradiation source 336 used to cure the fluid during recoating of the bareportion 250 of an optical fiber 20.

[0179] During its motion, the split die 334 applies a substantiallyuniform thickness of recoating composition along a length of fiber 20that includes the bare fiber portion 250 and margins at each end thatoverlap the original secondary buffer 102. Uniform coverage of anoptical fiber 20 with a concentric layer of a recoating compositionrelies upon the accuracy of positioning a filament organizer 10 topreferably place the fiber 20 coaxial with the sizing die 334. Theradiation cure chamber 336 has a size such that its internal surfaces donot touch the layer of recoating composition, either before or aftercuring. When coating concentrically, the bare fiber portion 250 willonly come into contact with the recoating composition. The splitconfiguration of the sizing die 334 and the radiation cure chamber 336allows easy positioning of a fiber 20 in a recoating fluid extrusionapparatus 330. Correct fiber positioning, as mentioned previously, is aresult of accurate engagement of a filament organizer 10 with arecoating fluid extrusion apparatus 330. Upon re-opening the die headassembly 332, after completing the recoating and curing process, a gapbetween a recoated fiber 20 and the internal surfaces of the radiationcure chamber 336 allows clean removal of the fiber 20 from the assembly332.

[0180] Changes in the length of bared fiber portions 250 may beaccommodated by adjustment of the distance that a die head assembly 332may travel while extruding recoating composition. The surface tension ofthe recoating composition tends to smooth out any irregularities in thecoating before it reaches the radiation cure chamber 336, even thoughthe die head assembly 332 has a length of only about 6.0 mm to about 7.5mm. A benefit of this short length is avoidance of contamination byrecoating composition. Also small amounts of residual recoatingcomposition may be relatively easily cleaned from inside the assembly332.

[0181] Although a bare fiber portion 250 has a horizontal orientationduring application of protective recoating composition, the movingextrusion die 334 produces similar results to coating heads operatingvertically during fiber draw coating processes. The relative motionbetween the sizing die 334 and the fiber 20 simulates the draw process.This eliminates mold recoating defects such as flash, gate marks, sinks,and coating delamination caused by coating adhering to the surface of amold.

[0182] The extrusion of terminal margins, at each end of the bare fiberportion 250, means that initial deposit of material by extrusion occursat a region of the fiber 20 that is still protected by the originalcoating 100, 102. This substantially prevents optical fiber strengthlosses generally associated with loading the fiber 20 into a traditionalrecoating mold. Bared fiber portions 250 recoated by split die extrusionaccording to the present invention provided evidence of strengthretention by surviving Vitran proof testing to levels exceeding 800kpsi.

[0183] A process for manufacturing an optical fiber Bragg grating hasbeen described to show how a compact filament organizer 10 may be usedto handle and transport optical fibers 20 between various types ofprocessing equipment. Each piece of processing equipment may include apair of mounting pins for alignment and insertion in through holes 80 ofa filament organizer 10 for correct positioning of a central portion ofan optical fiber 02 relative to the selected piece of apparatus. Sucheasy positioning also facilitates automation of at least parts of theBragg grating manufacturing process unlike previous similar processesthat rely upon operator skill for correct fiber positioning. It will beappreciated that engagement between mounting pins and through holes isonly one of a number of methods for aligning an optical fiber forprocessing.

[0184] As required, details of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary. Therefore, specific structural and functionaldetails disclosed herein are not to be interpreted as limiting, butmerely as a basis for the claims and as a representative basis forteaching one skilled in the art to variously employ the presentinvention.

What is claimed is:
 1. A filament coating apparatus comprising: a frameincluding a first filament holding fixture and a second holding fixturefor releasably securing a filament therebetween; an extrusion coatingfixture attached to said frame, said extrusion coating fixturecomprising: a guide rod; a carriage movably mounted on said guide rod; acoating head including an opening for directing a curable coatingcomposition to a filament positioned in a channel formed in said coatinghead and extending therethrough; and a radiation source aligned to saidcoating head for radiation curing of said curable coating compositionapplied to said filament, said coating head and said radiation sourcebeing joined to said carriage.
 2. A filament recoating apparatuscomprising: a planar surface; and an extrusion coating assembly attachedto said planar surface, said extrusion coating assembly comprising: afirst filament holding clamp; a second filament holding clamp oppositesaid first filament holding clamp to secure a measured filament portionincluding a bare portion thereof, located inside a first boundary and asecond boundary, between said first filament holding clamp and saidsecond filament holding clamp; a coating head including a die platehaving formed therein an open ended channel including a wall having afluid entry and a gas port formed therein adjacent a radiation source,said coating head further including a cover die plate having formedtherein an open ended elongate slot, said cover die plate having ahinged connection to said die plate for rotation of said cover die platebetween an open position and a closed position wherein said cover dieplate lies adjacent said die plate and said channel aligns with saidelongate slot to form a tubular opening through said coating head toencircle a section of said bare portion; a linear transport mechanismadjacent said coating head, said linear transport mechanism including aguide rod and a carriage slidably mounted thereon for movement alongsaid guide rod; and a connecting rod from said carriage to said coatinghead for linear displacement of said coating head during said movementof said carriage to move said coating head from said first boundary tosaid second boundary as a curable fluid flows from said fluid entry andenergy from said radiation source cures said curable fluid to recoatsaid bare portion of a filament.
 3. The filament coating apparatus ofclaim 2 wherein the filament is an optical fiber.
 4. A filament coatingapparatus comprising: a filament organizer having an extended filamentbetween a fixed spool and a rotary spool, said extended filament havinga measured filament portion and a bare filament portion; and anextrusion coating fixture adapted for attachment of said filamentorganizer, said extrusion coating fixture comprising: a guide rod; acarriage movably mounted on said guide rod; a coating head including anopening for directing a curable coating composition to said barefilament positioned in a channel formed in said coating head andextending therethrough; and a radiation source aligned to said coatinghead for radiation curing of said curable coating composition applied tosaid bare portion, said coating head and said radiation source beingjoined to said carriage.
 5. The filament coating apparatus of claim 4wherein the filament is an optical fiber.
 6. The filament coatingapparatus of claim 4, wherein said filament organizer further includes asupport having a first surface opposite a second surface and at least apair of through holes, said lockable spool and said rotary spool beingattached adjacent to said first surface.
 7. The filament coatingapparatus of claim 6, wherein said carriage includes at least a pair ofstuds for releasably mating with said at least a pair of through holesof said support to position said filament organizer on said carriage, toalign a filament in said channel for application of said curable coatingcomposition by said coating head and curing of said curable coatingcomposition by said radiation source.
 8. The filament coating apparatusof claim 7, wherein said measured filament portion has a first boundaryand a second boundary, said first and second boundaries located outsideof said bare portion to define the limits for applying said curablecoating composition along said measured filament portion.
 9. Thefilament coating apparatus of claim 4, further including a gas deliverytube directed towards said channel and mounted on said carriage adjacentto said coating head and said radiation source.
 10. A filament coatingapparatus comprising: a filament organizer having an extended filamentbetween a fixed spool and a rotary spool, said extended filament havinga measured filament portion and a bare filament portion; and anextrusion coating fixture adapted for attachment of said filamentorganizer, said extrusion coating fixture comprising: a guide rod; acarriage movably mounted on said guide rod; and a coating die includinga coating head and a radiation source, said coating head having anopening for directing a curable coating composition to said barefilament positioned in a channel formed in said coating die andextending therethrough, said radiation source aligned to said coatinghead for radiation curing of said curable coating composition applied tosaid bare portion, said coating die being joined to said carriage. 11.The filament coating apparatus of claim 10, further including a gasdelivery tube mounted in said coating die and opening to said channel.12. The filament coating apparatus of claim 10, wherein said coating dieis a split die having an open position and a closed position such thatin said open position said split die opens to receive a filament placedtherein and said closed position forms said channel having the filamentaligned with the axis thereof.
 13. A method for coating a filamentcomprising the steps of: providing a filament organizer having anextended filament between a fixed spool and a rotary spool, saidextended filament having a measured filament portion and a bare filamentportion; and attaching said filament organizer to an extrusion coatingfixture comprising: a guide rod; a carriage movably mounted on saidguide rod; and a coating die including a coating head and a radiationsource, said coating head having an opening for directing a curablecoating composition to said bare filament portion in a channel formed insaid coating die and extending therethrough; applying said curablecoating composition to said bare filament portion to provide a recoatedfilament portion; and exposing said recoated filament portion to saidradiation source for radiation curing of said curable coatingcomposition applied to said bare portion, said coating die being joinedto said carriage.